Wow, Thank you Chris.
That worked. I had to change the data type in Tableau on the data source page for the excel field to be a string. Changing the Data Extract [Cust No] to a number didnt solve it, but making both of them a string solved it. But for whatever reason changing it in excel directly didnt work.
Thank you so much Chris
I had a similar issue. This is how I solved it today:
Right click one of the data sources.
Click Edit Data Source...
Pull in the second data source to initiate the join.
In my case, I clicked the Inner Join icon because that pertained to me. You should be able to click the Left Join for your situation.
If Tableau correctly recognizes your left data field, choose it.
On the right side, click the dropdown box.
At the bottom, there should be an option titled Create Join Calculation...
In my case, I typed DATETIME([Data_Field_Name]) because I was having issues with dates. You'll want to use the format you need.
The red exclamation error icon should go away.
Let me know if you experience similar results.