You must use same data type while joining.
In your error says text and bigint.
So I think this is due to the ZIP field in each datasource being differently defined (looks like it's an INT [Integer] in one datasource and a String in the other.
If you are using a later version of Tableau, you can use the Join on calculation to sort this (from memory this came into T10.5)
Say the SQL table is the one with the INT
When you create the join, select ZIP from SQL Table, and then for the Excel side, select "Join on Calculated field" and create the calculation
bit of a guess, but that might do the trick
Hi Seb as Sankar and Simon said 2 different data types either fix it in excel (change the zip to number format) or try what Simon said in Tableau.
Another way would be to load them as 2 data source and join them as a relationship in the data preferences, above would be much easier than this route though!
To create a join, your tables should come from one data source. Here, your one source is excel and another MS SQL Server so you are getting error. Load the excel data in sql server then you will be able to create join.
Hope it helps!