Not sure if this is the right thread, but I was trying to install Catalina on my macpro5,1(updated 4,1) using the "install on a supported mac first" method, and I got the prohibitory mark when trying to boot.
Here's the story: I downloaded a full copy from the developer SUCatalog using the installinstallmacos.py script, and installed on an SSD, attached to my iMac14,2. The update went well but, but,weirdly, I found out that my mac was enrolled to receive developer updates although I am not enrolled to the developer program.
I then moved the SSD to my macpro and the mac refused to boot: I was getting the prohibitory mark after selecting it. I had to install back the GT120 to find this out, as with my MSI Radeon Vega56 I couldn't see anything, of course.
As a side note: I decided to try to update the OS on the SSD, since the iMac was suggesting to update to 10.15.3 Beta2; update completed and now bootrom on the iMac is 141.0.0.0, and there is also a new default font.
Any idea of why the MacPro reports the prohibitory sign ? Is it because of this developer enrollment ?
Or maybe because a bootrom update was triggered and this was obviously not compatible with the MacPro?