Hard drive upgrade not working?

iMac 2015, 8Gb, 1TB. Running Mac OS X Mojave. Used iFixit hard drive upgrade kit with Samsung 860 Evo 1TB SSD.

Opened in 10 minutes, disk pout in 5. Toughest bit was getting the new adhesive strips fitted - took ages to line up!

Now the tough bit.

I had cloned the drive, had to format as APFS, as when I did it as Mac OS Extended (Journaled) didn’t boot, and so I had to redo the clone.

Boots fine from USB port, bit when in the iMac, won’t boot - instead I get a folder icon with “?” in the middle. Reinstalled Had drive, and it went fine, so no probs with iMac. Also, rebooted from SSD with no issues.

Tried in iMac again, still no luck. Now back with Hard disk, and screen held on with tape.

What to do?