The columns used in the join are not all the same data type.
In Legacy connections, the Microsoft JET Database Engine is responsible for determining the data types for each column in your data source.
Note: Starting in Tableau 2019.1, you can no longer create a new Legacy Excel or Text connection. Existing workbooks created with these legacy connections will continue to work until 2019.2, when all support for legacy connections will be removed. See the Legacy Connection Alternatives document in Tableau Community for alternatives to using the legacy connection.