An activity has been completed, but it won't let me close the activity complete

See attachment as it shows there is 4 hours of work, but we have correctly added the 4 hours on Real Time Allocation.

Hem...
Clearly not.
Your print screen clearly shows :
assigned : 4
real : 0
left : 4

Please post screenshot of real work allocation to see what's wrong.

I agree, this is strange.
There is some unconsistency betwwen assignement (real=4) and activity (real=0)
I cannot explain this, as real on activity is sum of real on assignments...

1) Do you have any errors in log file ?
2) Can you try to make a change in assignement ? (for instance, add 1 hour of left work, save, then revert change to 0 hours left) Does it fox the issue ?
3) which version of projeqtor are you using ?

Where do I find this to check?

Default location is in projeqtor/files/log

I tried this and it worked and it removed the hours left and displayed it as 0.

Great !

Still using 6.1.2

This is not the last patch on V6.1.
You should at least migrate to V6.1.3
But my advise is to migrate to V6.4.4 (every version brings fixings, sometimes to old isues)