Ich sah den ersten Rechner ?
Yes, that's fine. Report it if it wasn't accepted.
I've reported this one too
can you use simple past with this sentence - "sah" instead of "habe gesehen"?
Yes, you can.
With a few exceptions (like "sein"), simple past is restricted to written German.
People always say this, but I've heard loads of Germans use the simple past! I think German speakers use it more than they think.
Nicht wahrscheinlich, Alan Turing hat den ersten Computer erfunden 1936!