Your question cannot be answered from the picture of the model and there is no way to verify that you have correctly described what you have actually done based on what you have shown. I do not believe that the alias would cause the failure. Anyway, you should not use the Rename option and instead should use the Properties dialog to change the Alias to match your field name to eliminate the confusion of having two fields with the same alias.
Since the calculation is on a joined table you need to fully qualify the table names for the fields in your calculation. You cannot make the expression just [approval_rate] on a joined table. Provide a screen shot of the calculate Field (2) set up so we can verify that the expression has been done correctly for the join you have created.