This error states that the Tax field on different custom forms are not the same. It also lists all the names of the custom forms followed by the name of the tax field that was renamed. For example, "Form Name: Custom Invoice Form Field Name: Custom Tax Label".
1. Navigate to Customization > Forms > Transaction Forms > Edit Custom Invoice Form
2. Click the Sublist Fields Subtab.
3. Rename Custom Tax Label to Tax
4. Click Save
Note: Repeat the same steps for other custom forms listed in the error message until all tax fields are named the same. Please note that if you cannot see a tax field on the Sublist Fields you can also check the Printing Fields > Footer to see if the Tax field there was re-labelled.