[Google Pixel 4 XL Guides, News, & Discussion] Santhosh_Nagendra:
Yes. Even I am also stuck with same issue :

RootDevice_GooglePixel4a\platform-tools_r31.0.3-windows (2)\platform-tools>fastboot reboot bootloader

Rebooting into bootloader OKAY [ 0.053s]

Finished. Total time: 0.061s

RootDevice_GooglePixel4a\platform-tools_r31.0.3-windows (2)\platform-tools>fastboot flash bootloader --slot all

unknown partition 'bootloader'

fastboot: error: cannot determine image filename for 'bootloader'

RootDevice_GooglePixel4a\platform-tools_r31.0.3-windows (2)\platform-tools>fastboot flash bootloader --slot all bootloader-bramble-b5-0.4-7758094.img

Sending 'bootloader_a' (8762 KB) OKAY [ 0.318s]

Writing 'bootloader_a' (bootloader) Flashing Pack version b5-0.4-7758094

(bootloader) Flashing partition table for Lun = 0

(bootloader) Flashing partition table for Lun = 1

(bootloader) Flashing partition table for Lun = 2

(bootloader) Flashing partition table for Lun = 4

(bootloader) Flashing partition table for Lun = 5

(bootloader) Flashing partition xbl_a

(bootloader) Flashing partition xbl_config_a

(bootloader) Flashing partition aop_a

(bootloader) Flashing partition tz_a

(bootloader) Flashing partition hyp_a

(bootloader) Flashing partition abl_a

(bootloader) Flashing partition keymaster_a

(bootloader) Flashing partition devcfg_a

(bootloader) Flashing partition qupfw_a

(bootloader) Flashing partition uefisecapp_a

(bootloader) Flashing partition featenabler_a

(bootloader) Flashing partition logfs

OKAY [ 0.388s]

Sending 'bootloader_b' (8762 KB) OKAY [ 0.317s]

Writing 'bootloader_b' (bootloader) Flashing Pack version b5-0.4-7758094

(bootloader) Flashing partition table for Lun = 0

(bootloader) Flashing partition table for Lun = 1

(bootloader) Flashing partition table for Lun = 2

(bootloader) Flashing partition table for Lun = 4

(bootloader) Flashing partition table for Lun = 5

(bootloader) Flashing partition xbl_b

(bootloader) Flashing partition xbl_config_b

(bootloader) Flashing partition aop_b

(bootloader) Flashing partition tz_b

(bootloader) Flashing partition hyp_b

(bootloader) Flashing partition abl_b

(bootloader) Flashing partition keymaster_b

(bootloader) Flashing partition devcfg_b

(bootloader) Flashing partition qupfw_b

(bootloader) Flashing partition uefisecapp_b

(bootloader) Flashing partition featenabler_b

(bootloader) Flashing partition logfs

OKAY [ 0.432s]

Finished. Total time: 2.100s

RootDevice_GooglePixel4a\platform-tools_r31.0.3-windows (2)\platform-tools>fastboot reboot bootloader

Rebooting into bootloader OKAY [ 0.040s]

Finished. Total time: 0.056s

\RootDevice_GooglePixel4a\platform-tools_r31.0.3-windows (2)\platform-tools>fastboot flash radio --slot all radio-bramble-g7250-00161-211008-b-7807492.img

Sending 'radio_a' (149780 KB) OKAY [ 3.665s]

Writing 'radio_a' (bootloader) Flashing Pack version SSD:g7250-00161-211008-B-7807492

(bootloader) Flashing partition modem_a

OKAY [ 0.824s]

Sending 'radio_b' (149780 KB) OKAY [ 3.636s]

Writing 'radio_b' (bootloader) Flashing Pack version SSD:g7250-00161-211008-B-7807492

(bootloader) Flashing partition modem_b

OKAY [ 1.085s]

Finished. Total time: 9.842s

RootDevice_GooglePixel4a\platform-tools_r31.0.3-windows (2)\platform-tools>fastboot reboot bootloader

Rebooting into bootloader OKAY [ 0.044s]

Finished. Total time: 0.053s

RootDevice_GooglePixel4aplatform-tools_r31.0.3-windows (2)\platform-tools>fastboot update -w image-bramble-sq1a.211205.008.zip --skip-reboot --slot all

Warning: slot set to 'all'. Secondary slots will not be flashed.

--------------------------------------------

Bootloader Version...: b5-0.4-7758094

Baseband Version.....: g7250-00161-211008-B-7807492

Serial Number........: 15191JECB01734

--------------------------------------------

extracting android-info.txt (0 MB) to RAM...

Checking 'product' OKAY [ 0.075s]

Checking 'version-bootloader' OKAY [ 0.056s]

Checking 'version-baseband' OKAY [ 0.072s]

Setting current slot to 'a' OKAY [ 0.145s]

extracting boot.img (96 MB) to disk... took 0.244s

archive does not contain 'boot.sig'

Sending 'boot_a' (98304 KB) OKAY [ 2.440s]

Writing 'boot_a' OKAY [ 0.679s]

archive does not contain 'boot.sig'

terminating with uncaught exception of type std::out_of_range: basic_string