Your command is wrong.
The correct one would be:
Fastboot flash boot magisk_patched.img
i did that earlier.... just did it again... and it won't boot after doing that.... says "error boot prepare" (on fastboot screen)
i had repatched the lineage-17.1-20200915-recovery-crosshatch.img in magisk manager again to be sure.... and just tried it again....
my phone won't boot up again now... i guess i'll just leave it this way for a bit instead of reflashing the unpatched lineage recovery (to get it to boot up again).... in case there is something i am missing here
thanks for any help