I've entered info into What-if worksheet, but it doesn't calculate anything. How do I ask it to calculate?

It will calculate automatically when it's filled in correctly. Did you enter the filing status and age at the top of the column? It can't calculate without that information.

Try copying column 1 to one of the other columns (use the "copy columns" boxes at the top) and modify it, instead of filling in the whole column from scratch.

Thank you. That’s what I was not doing. I did not enter the filing status. Works fine now

Solved in comments.