I withdrew 20K from my 401K but paid it back in 30 days. How do I enter this?

That's considered a rollover.  Enter the 1099-R showing the withdrawal, and then you should be asked some questions about what you did with the money.  Indicate that you rolled it over into the same or another qualified plan. 

Did they take any tax withholding out of it?  If they did you needed to put back the full gross amount including the withholding or the withholding is a taxable distribution.  

It depends. If you took what would have been a taxable withdrawal, and paid it back within 60 days, then you can indicate that in the 1099-R entry.

If you just took a 401(k) loan, not a withdrawal, there is nothing to report.

Here is some information you may find helpful: An Early Withdrawal From Your 401(k): Understanding the Consequences.

[Edited 02/15/2025 | 3:50 pm PST]