This running from a subject area in your RPD or direct SQL?

ORA-00937

Error ORA-00937 occurs when the GROUP BY command is improperly used. The GROUP BY command allows the user to view rows having a common field value in a single row. For example, a small business CEO wants to view employees who worked the highest number of hours by department last month. Another instance could be when a school superintendent wants to view students on the honor roll by homeroom. In such cases, the user may run a GROUP BY statement to view the results.

GROUP BY is used in conjunction with aggregate functions to filter the results by a value. The GROUP BY command can be very useful for viewing a select set of results. It is commonly used, which deems it essential to utilize the proper syntax when running the statement.

A SELECT list cannot include both a group function, such as AVG, COUNT, MAX, MIN, SUM, STDDEV, or VARIANCE, and an individual column expression, unless the individual column expression is included in a GROUP BY clause.

In other words, you tried to execute a SELECT statement that requires a GROUP BY clause without including the GROUP BY clause. If you are using an aggregate function in your select query (e.g. AVG, COUNT, MAX, MIN…), you must have a GROUP BY clause.

then it's time to take a look at the RPD ... specifically content levels ...