Cannot apply more than 256 MB graphic memory to Mojave 10.14.6 VM

It seems that there could be an issue with the graphics card compatibility with the Parallels driver. You may try updating the driver for your Radeon Pro 580 8GB to the latest version. You may also reach out to Parallels support for more assistance on the compatibility issue. They may have specific troubleshooting steps or workarounds for your setup.