Error “Unable to run diagnostics DEC500” on iMac 5K 2014 with OpenCore

Hi, I’m trying to upgrade my iMac (Retina 5K, 27”, Late 2014) using OpenCore to install a newer macOS version, but when selecting the USB EFI, I get the error “Unable to run diagnostics DEC500”.

I successfully created the macOS installer, but this error prevents me from proceeding. Has anyone encountered this issue or knows how to fix it?

iMac specs: • iMac 5K (Late 2014) • Intel Core i5 3.5 GHz • 8GB DDR3 • AMD Radeon R9 M290X 2GB • Current OS: macOS Yosemite (10.10.5)

Any advice would be greatly appreciated. Thanks!