Can't get "Startup Manager" to launch using the held "alt" key at power on... Black screen, no boot

I have been using Mac minis for 15 plus years, and have confidently used SuperDuper to make a bootable backup for…. A long time, but In November of last year, I bought a new 2018 mini ( MRTR2LLA ... Macmini8,1 ... No. A1993 ) and I waited about 2 months before I got an Apple tech to walk me through installing Catalina… A lot to learn about the new OS… but I like it… Then I made a SuperDuper, bootable backup… But when I tried to boot it using “alt” at power on, it didn't launch “Startup Manager”… in fact it didn't do anything… black screen… for a long long time.. NOTHING…. I've tried Mac OS Extended (Journaled) and APFS (both of which will boot if set up in System Preferences/Startup Disk) but nothing I've tried will bring up the 'Startup Manager” … I even tried 3 wired keyboards, in case it was a key failure, but the results were all the same... So, I've lost my confidence in the new mini…. I even reinstalled Catalina, but it didn't help... What's next ..??