Q: if my computer can't show the serial number in this window, where else can i find the serial number?

What happens when you click the Version text?

If it is not showing up there, did you ever have the logic board replaced?

If it doesn't show there, click on More Info. It should be there on the Overview page.

The other place to find the serial number is in the System Information app. Hold down the option (alt) key and select the Apple Menu. Choose System Information. If it is not already selected, select the Hardware item in the list on the left. In the Hardware Overview, the serial number is towards the bottom of the list.

Click the More Info button.

I have heard that a logic board replacement will make that happen, but I don't know about the other stuff.

If you've got the box it came in, I think the serial number is printed on the label somewhere.

I didn't say anyone changed your logic board. That is the only thing I've heard of that will do that. The other stuff you did might have done it, but I don't know.

Was it a refurbished model?