You responded that you are using the desktop version.
As I said above, it's probably a good idea to make a copy of your *.tax2018 data file and use it for the experimenting. Be sure you name that copy something completely different, so it doesn't overwrite your original return. For instance "testreturn.tax2018" or something. You could make a backup copy of your original return on backup media, external storage, etc.
When you open the new copy of the 2018 return, if you want to use the "What If" worksheet, go into Forms Mode. Then above the list of forms in the left column there is a forms search tool called OPEN FORM. Click that and enter the word "what" without quotes, or "what-if" without quotes but with the hyphen. Then select the worksheet.
Or if you made a separate test copy of the data file to experiment with, you can actually experiment in the interview mode as well.
