1989 Supra will not start

[AlmostRunning]

Just finished a 4 month long MHG replacement with my son. It was a project to for us to do together and for both of us to learn as we went along. Now we are so close to the end, but getting very frustrated. After closing up the engine after the rebuild it started on first try and then died after a few minutes. Replaced Distributor, reset computer and it started and ran. Took for a test drive and it ran fine. Shut it down and now it will not start. Prior to replacing distributor when it would not start the first time, I tested for spark and no spark. Replaced coil-nothing. After replacing distributor, it started fine. Now, again, no spark. Prior to replacing dis again and having the same results, I am trying to find out from the experts here if there is anything else that could be wrong. Tested resistance for distributor and AFM and both were within tolerance. Wondering if a no-start "signal" is being sent from computer. Why would it start and run fine and then not start? Have tried to follow the TSRM, but am stuck. Have rechecked all the grounds and they are secure. Battery voltage is 12+ and getting 12+ on the grounds too (engine/firewall/grounding points). Timing set while running - no problems. Only codes is 12 and 51 and not sure if they is related or not. Tried to follow TSRM on the codes, but can't seem to get anywhere.

1989 Supra mkiii non-turbo manual

Thanks for helping.

Almostrunning