First make sure this is not an informational item only. If your K-1 shows gains from sales of business property then it's an actual reporting item.  Typically the 1250 gain is shown on Box 8c if an S corp and 9c if a partnership and that automaticaly flows. Box 20 doesn't flow from the K-1 to the proper form so if actual then you have to go to the Unrecaptured Secton 1250 Gain Worksheet and adjust it there.

At the bottom of Sch D theres a button to the Unrecaptured 1250 Gain Worksheet, but Im not sure how to get the numbers to flow to it, there is an Enterable 4797 (use Where Do I Enter and type 4797, you'll see the Enterable form listed), but I fiddled around with it a bit but I dont see a place to just stuff a number into it, it wants more info than that.

I can override the amount in here and it does with it should, but I feel like there should be a way to do it without an override using the 4797.

I agree with dascpa - Box 20 might not actually affect anything.  Is the amount also in Box 9c?

You may need to get more information about why it is listed in Box 20.