I left a job in 2020. I received a check for the money that I had in my 401k as of my last day. Taxes were taken out of the amount. How do I go about reporting this?

The plan administrator should issue a 1099-R for the distribution, you might want to check with them why you did not receive a 1099-R.

If you cannot get a 1099-R issued then you will need to create a substitute 1099-R:

1. Login to your TurboTax Account 
2. Click on "Search" on the top right and type “1099-R”
3. Click on “Jump to 1099-R”
4. Enter information in the 1099-R fields
5. Continue until "Do any of these situations apply to you?" screen and check "I need to file a substitute 1099-R"

You need to get the 1099-R from them.