There are several weird aspects to the UI for Formula fields in Custom Modules that should be addressed.

First, and likely most fixable, is the requirement of the syntax checker that the output be a string. Specifically, the issue is that the user is required to format the field as a string (with zero guidance as to how to do that).

The most common use for a formula field is likely going to be quick arithmetic of numbers or decimals. The output from that is obviously going to be a number or a decimal.

When I tried that and checked the syntax, I got this error:

I looked in the Functions section, and there was no .toString function or anything of the like. I figured out that if you enclose the entire statement in double quotes (" "), that satisfies the syntax checker.

I figured out that if you save the formula without checking the syntax, it works just fine. However, the syntax checker shouldn't flag something as an error that isn't.

Along those same lines, the syntax checker doesn't ensure that spacing is correct. It passes field_name1/field_name2, but that doesn't render properly. It needs to be field_name1 / field_name2 (with spaces between the field name and operator).

The other problem is that the fields added to the Custom Module aren't available (at least not right away) to add to the formula. Sure, they're added after the module is saved, but that's a weird UX. Zoho should figure out how to have all fields added to the Custom Module immediately available in the Fields drop-down menu in Formulas.