[One (M7) Q&A, Help & Troubleshooting] nkk71:
copy/paste your command prompt output (it's really hard to guess when the info is not provided), for example:

C:\ADB5_1.0.32>fastboot reboot-bootloader
rebooting into bootloader...
OKAY [ 0.040s]
finished. total time: 0.040s

C:\ADB5_1.0.32>fastboot oem rebootRUU
...
(bootloader) Start Verify: 0
OKAY [ 0.050s]
finished. total time: 0.060s

C:\ADB5_1.0.32>fastboot flash zip 7.15.401.1-Combined_TWRP2840_NoWipe.zip
target reported max download size of 1514139648 bytes
sending 'zip' (40953 KB)...
OKAY [ 3.490s]
writing 'zip'...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) start image[hboot] unzipping for pre-update check...
(bootloader) start image[hboot] flushing...
(bootloader) [RUU]WP,hboot,0
(bootloader) [RUU]WP,hboot,99
(bootloader) [RUU]WP,hboot,100
(bootloader) ...... Successful
FAILED (remote: 90 hboot pre-update! please flush image again immediately)
finished. total time: 5.100s

C:\ADB5_1.0.32>fastboot flash zip 7.15.401.1-Combined_TWRP2840_NoWipe.zip
< waiting for device >
target reported max download size of 1514139648 bytes
sending 'zip' (40953 KB)...
OKAY [ 3.478s]
writing 'zip'...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) start image[sbl1-1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-1,0
(bootloader) [RUU]UZ,sbl1-1,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[sbl1-2] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-2,0
(bootloader) [RUU]UZ,sbl1-2,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[sbl1-3] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-3,0
(bootloader) [RUU]UZ,sbl1-3,100
(bootloader) signature checking...
(bootloader) [RUU]WP,sbl1-3,0
(bootloader) [RUU]WP,sbl1-3,100
(bootloader) ...... Successful
(bootloader) start image[sbl1-4] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-4,0
(bootloader) [RUU]UZ,sbl1-4,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[sbl1-5] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1-5,0
(bootloader) [RUU]UZ,sbl1-5,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) start image[sbl2] unzipping & flushing...
(bootloader) [RUU]UZ,sbl2,0
(bootloader) [RUU]UZ,sbl2,100
(bootloader) [RUU]WP,sbl2,0
(bootloader) [RUU]WP,sbl2,100
(bootloader) ...... Successful
(bootloader) start image[sbl3] unzipping & flushing...
(bootloader) [RUU]UZ,sbl3,0
(bootloader) [RUU]UZ,sbl3,100
(bootloader) [RUU]WP,sbl3,0
(bootloader) [RUU]WP,sbl3,100
(bootloader) ...... Successful
(bootloader) start image[tp] unzipping & flushing...
(bootloader) [RUU]UZ,tp,0
(bootloader) [RUU]UZ,tp,100
(bootloader) ..... Bypassed
(bootloader) start image[tp] unzipping & flushing...
(bootloader) [RUU]UZ,tp,0
(bootloader) [RUU]UZ,tp,100
(bootloader) ...... Successful
(bootloader) start image[tz] unzipping & flushing...
(bootloader) [RUU]UZ,tz,0
(bootloader) [RUU]UZ,tz,100
(bootloader) [RUU]WP,tz,0
(bootloader) [RUU]WP,tz,100
(bootloader) ...... Successful
(bootloader) start image[adsp] unzipping & flushing...
(bootloader) [RUU]UZ,adsp,0
(bootloader) [RUU]UZ,adsp,15
(bootloader) [RUU]UZ,adsp,27
(bootloader) [RUU]UZ,adsp,37
(bootloader) [RUU]UZ,adsp,49
(bootloader) [RUU]UZ,adsp,60
(bootloader) [RUU]UZ,adsp,72
(bootloader) [RUU]UZ,adsp,82
(bootloader) [RUU]UZ,adsp,94
(bootloader) [RUU]UZ,adsp,100
(bootloader) [RUU]WP,adsp,0
(bootloader) [RUU]WP,adsp,100
(bootloader) ...... Successful
(bootloader) start image[cir] unzipping & flushing...
(bootloader) [RUU]UZ,cir,0
(bootloader) [RUU]UZ,cir,100
(bootloader) ...... Successful
(bootloader) start image[pg2fs_spcustom] unzipping & flushing...
(bootloader) [RUU]UZ,pg2fs_spcustom,0
(bootloader) [RUU]UZ,pg2fs_spcustom,22
(bootloader) [RUU]UZ,pg2fs_spcustom,48
(bootloader) [RUU]UZ,pg2fs_spcustom,70
(bootloader) [RUU]UZ,pg2fs_spcustom,97
(bootloader) [RUU]UZ,pg2fs_spcustom,100
(bootloader) ...... Successful
(bootloader) start image[recovery] unzipping & flushing...
(bootloader) [RUU]UZ,recovery,0
(bootloader) [RUU]UZ,recovery,8
(bootloader) [RUU]UZ,recovery,17
(bootloader) [RUU]UZ,recovery,25
(bootloader) [RUU]UZ,recovery,30
(bootloader) [RUU]UZ,recovery,38
(bootloader) [RUU]UZ,recovery,47
(bootloader) [RUU]UZ,recovery,68
(bootloader) [RUU]UZ,recovery,73
(bootloader) [RUU]UZ,recovery,81
(bootloader) [RUU]UZ,recovery,86
(bootloader) [RUU]UZ,recovery,94
(bootloader) [RUU]UZ,recovery,100
(bootloader) [RUU]WP,recovery,0
(bootloader) [RUU]WP,recovery,100
(bootloader) ...... Successful
(bootloader) start image[rpm] unzipping & flushing...
(bootloader) [RUU]UZ,rpm,0
(bootloader) [RUU]UZ,rpm,100
(bootloader) [RUU]WP,rpm,0
(bootloader) [RUU]WP,rpm,100
(bootloader) ...... Successful
(bootloader) start image[radio] unzipping & flushing...
(bootloader) [RUU]UZ,radio,0
(bootloader) [RUU]UZ,radio,5
(bootloader) [RUU]UZ,radio,11
(bootloader) [RUU]UZ,radio,17
(bootloader) [RUU]UZ,radio,23
(bootloader) [RUU]UZ,radio,29
(bootloader) [RUU]UZ,radio,34
(bootloader) [RUU]UZ,radio,40
(bootloader) [RUU]UZ,radio,46
(bootloader) [RUU]UZ,radio,52
(bootloader) [RUU]UZ,radio,58
(bootloader) [RUU]UZ,radio,63
(bootloader) [RUU]UZ,radio,69
(bootloader) [RUU]UZ,radio,75
(bootloader) [RUU]UZ,radio,81
(bootloader) [RUU]UZ,radio,86
(bootloader) [RUU]UZ,radio,92
(bootloader) [RUU]UZ,radio,99
(bootloader) [RUU]UZ,radio,100
(bootloader) [RUU]WP,radio,0
(bootloader) [RUU]WP,radio,23
(bootloader) [RUU]WP,radio,46
(bootloader) [RUU]WP,radio,69
(bootloader) [RUU]WP,radio,92
(bootloader) [RUU]WP,radio,100
(bootloader) ...... Successful
(bootloader) start image[rcdata] unzipping & flushing...
(bootloader) [RUU]UZ,rcdata,0
(bootloader) [RUU]WP,rcdata,0
(bootloader) ...... Successful
OKAY [ 58.061s]
finished. total time: 61.539s

C:\ADB5_1.0.32>fastboot reboot-bootloader
rebooting into bootloader...
OKAY [ 0.040s]
finished. total time: 0.040s

C:\ADB5_1.0.32>

did you leave a space between reboot and RUU; it's one word

fastboot oem rebootRUU