[Nexus 7 (2013) General] geckocavemen:
Based on what you said about the internal storage reporting as 0 mb it sounds like you ran into a soft-brick issue that can occur when the tablet isn't restarted and connected to the internet after you complete the unlocking process (part B of the guide). The good news is your tablet isn't broken, but it is going to take a little bit of work to get it going again.

First, see if you can get the tablet to boot to the homescreen. If it does boot all the way, download SuperSu from the link in the guide and then reboot into TWRP and install the SuperSu file you just dowloaded and you are good to go.

If it won't boot, go into TWRP, select Wipe --> Advanced Wipe --> choose Data and then swipe to wipe. Now try to boot the tablet and if it does boot follow the directions in the above paragraph.

If none of that works and you are still stuck and can't get to the homescreen you will have to manually flash the factory image from Google to restore the original OS. I've done it before and I'll provide instructions below:

1. On your Mac, download the factory image for the 2013 Nexus 7 here (make sure you get the most recent factory image for "razor" which is the wifi version of the nexus 7 2013)

2. The file that will download will be a .tgz file. Once the download is done, unzip the .tgz file

3. Open the folder that unzipped and unzip another file in the new folder that should be called image-razor-kot49h.zip

4. After that unzips you'll get another folder called image-razor-kot49h. There will be 6 files in that folder. Take the boot.img, cache.img, system.img, and userdata.img files and place them in your platform-tools folder that is located in the android sdk you downloaded earlier for the rooting process. The platform-tools folder is also where you find the adb and fastboot files.

5. Boot the Nexus 7 into fastboot mode (this is where you can see the green "Start" at the top and also see all the information about your tablet in the lower left part of the screen) and plug it into your computer

6. Open terminal and navigate to your platform-tools folder using the "cd" command like you did in the tutorial (type cd into terminal and then drag and drop your platform-tools folder onto terminal and press enter)

7. To make sure your tablet is recognized and you navigated to the correct folder type the following into terminal and press enter (make sure the serial number of your tablet shows up in terminal): ./fastboot devices

8. Type the following into terminal and press enter (this might take a minute or two to complete): ./fastboot flash system system.img

9. Type the following into terminal and press enter (this should complete very quick): ./fastboot flash userdata userdata.img

10. Type the follwoing into terminal and press enter (should complete very quick): ./fastboot flash boot boot.img

11. Type the following into terminal and press enter (should complete very quick): ./fastboot flash cache cache.img

12. Type the following into terminal and press enter: ./fastboot reboot

13. Your tablet should now be reset to its original factory state and it should reboot. However, it will still be unlocked and if you want to root it you can begin with section C. of the guide. Just make sure you connect to the internet first and use the link provided in the guide to download the SuperSu file to your tablet. If you don't connect to the internet and download the SuperSu file first, TWRP will still attempt to install SuperSu even though you haven't downloaded it and you will run into the same soft-brick issue you just had.

Hope this helps and let me know if you still have issues.