Only the Desktop program has a What-if worksheet and allows you to copy your return with another name to test in. You can always do a what if as long as you do NOT use your real return to test in. You need to set up a new account. Don't touch your real return or that will change it and you won't be able to get a copy.

The online versions are only good for 1 return but you can use the same email address for 5 accounts. See the FAQ below to start another return in the online version:

https://ttlc.intuit.com/replies/3288574