Unable to Save Formula Due To an Error

Possible causes are as follows.

Field Code Is Not Used in Formula

You must use field code of each field in your formula.
You cannot use "field names" in your formula.

Click the setting icon Setting icon for the target field, and then click Settings to open the field setting screen. At the bottom of the screen, check the "field code" displayed.
If you have not specified the field code in your formula, specify it.

Formula Is Not Correct

You must use half-width parenthesis in your formula.
When setting a function, make sure that full-width parenthesis is not used and that you have not forgot any required parenthesis.

Example: SUM(Subtotal)