Big sur wont install

I have a working hack with both Mojave and Catalina installed on separate SSD's.
I have created an opencore Big Sur USB installer, however my computer does not recognise it as a boot drive.
I have checked it on my macbook and it is bootable.
I ran into this issue on my mobo a few years ago and solved it by unplugging everything and putting in the stick after the bios screen went, however I cannot get it to work now.
Any suggestions?

Author: paolo