Can an error message appear when the sum of 2 entered values is over a set maximum?

Yes, that would be possible to do.

To do it, you would simply need to add:

1 form calculation to your form that will do the sum of the two fields

1 condition that will check if that value is under the maximum.

The following guides will help you see how to do the calculations:

How to Assign Calculation Value

How to perform calculation in the form?

And the following with the condition:

How to show or hide a field in the form based on the other field input or selection

Of course, if you happen to have any issues setting it up, just let us know.