Android 11 root process shows "no valid slot to boot" after flashing patched.img

This is the first time I’ve rooted a phone. I have a Google pixel 4a, running Android 11. I use Ubuntu 20.04 and have a limited knowledge of the command line.

I followed the process at this xda-link fairly smoothly until the last step before confirming root.

After extracting the stock boot.img, patching it with magisk manager, and running the command:

Fastboot flash boot magisk_patched.img

The Google logo pops up for about 10 seconds and then kicks me back out to fastboot mode. The "reason" displayed on the fastboot menu screen after this says:

No valid slot to boot

Im lost as to why this is happening, and I cant seem to find many people with similar issues. Which leads me to believe that I’ve missed something simple. Thank you for any insight, it is greatly appreciated.