How can I enter the asset cost basis on line 21 oof pg. 2, form 4797?
The entry on that Line gets there through Step-by-Step (the interview section) in Business assets or Rental assets.
Did you proceed through Step-by-Step?
Also, which type of return are you preparing?
Not sure you understand that line 21 is populated from another form. I can’t just enter the number
I have done that step by step
You can actually enter a figure directly on the form in Forms Mode.
If you need help, you will have to provide more details with respect to the transaction you are trying to enter.