You absolutely can join to a calculated field, but they make you take a few extra steps:
After you drag your source over, and the dialogue box pops up, under Data Source, you will see all of your field choices but none of your calculated fields. At the bottom of these choices is Edit Join Calculation; click on it.
A Join Calculation box pops up. Type in the name of your calculated field. Put the name in [ ]'s, or after typing a few letters, the name should pop in as an auto-fill option. Once done, click ok.
Then click on the second column and choose the corresponding field you want to use for the join. At that point, it should perform the join.
For an in-depth explanation check out Tableau Training Course details.