Worked but windows is jacked.You may need to clear NVRAM. Windows probably took over the boot path. If you have an EFI video card boot with option key and select opencore drive with control+enter and from there boot Windows.
I did the NVRAM reset. Then put macOS OC drive back in and turned computer on. Windows drive appeared on the Open Core boot screen....SWEET! But when I select the windows drive, windows gives me the blue screen of death....Not so sweet . Drive must have become corrupted from the legacy->EFI conversion. So it's probably still not a good thing to get the error during the initial cmd prompt efi conversion.
EDIT: Ive used RX 580 the entire time.
I finally got the windows drive to show up on the opencore bootscreen and for windows to boot normally. These are the additional steps I did to fix my issues:
1. Remove windows drive, insert macOS opencore drive and do a NVRAM reset (hold option, cmd, P, & R keys for 20 seconds upon booting).
2. After the NVRAM reset reinsert windows drive. Boot and opencore bootscreen appeared with windows drive.
3. For me the initial boot into windows gave me the blue screen of death. Boot windows into SAFE MODE. Restart and it should all work normally with the OC boot screen and windows booting normally!