Row level (which yours is) and aggregate calculated fields are usually* calculated as part of the queries generated by Tableau, which are based on your data connection, including joins. Therefore, you will not be able to use them as part of a join. What is your source? If you are able to perform the calculation in the source then you will be able to join. Or you might consider using a custom SQL statement which concatenates the fields for the join. Depending on what you need from the joined table, Data Blending might be an option to consider. In the case of data blending, you can use a calculated field to link the tables.
*Row level calculations may be materialized in extracts.