I was reviewing my tax return and noticed Line 7 said 137,700. I did not make that much last year! What's going on??

Deleted answer

That is just a prefilled amount for the max amount subject to Social Security Tax.  If your total W2 and SE income is more than that you don't pay SS tax on the amount over.  If your income is less then box 7 doesn't figure into it.

See it's even on the blank IRS SE form

https://www.irs.gov/pub/irs-pdf/f1040sse.pdf

So it's not saying I made that much?

Right.  That is not how much you made.  That is just the MAX amount the SE tax is based on if you made more than 137,700.    Don't worry about it.   It is on everyone's return.