Messages of the following form:
Fatal error: Call to undefined function in \administrator\components\com_rsform\helpers\rsform.php : eval()'d code on line 1
are being generated by the fact that PHP has detected an error in the syntax of your custom PHP code that you have added in either one of RSForm!Pro's PHP Scripts areas (Script called on form display, Script called on form process, Script called after form has been processed), PHP Email Scripts areas (Script called before the User Email is sent, Script called before the Admin Email is sent and Script called before the Additional Emails are sent), as well as the Manage Directories's PHP Scripts areas (Scripts called on listing layout, Scripts called on details layout and Script called before the Directory Emails are sent).
To fix this, simply check your PHP custom code and fix its syntax errors. To make sure you write the code correctly, please have a look at the PHP reference.
If you are getting the same type of error when using custom PHP code inside a field's Default value area or in a select field's Items area, you have probably already noticed that you will no longer be able to access your website's backend area to correct the code. In this situation, you will need to change the values set for these areas directly in the database tables, specifically in the _rsform_properties table.