Aggregate function doesn't work with non-aggregate

Hello,

I've pulled my data from Zoho Projects and I am trying to work on creating reports for the hours that have been performed. Specifically, I'm trying to show the remaining hours for a project on the pivot table.

So, I'm taking the Budget Hours from Projects table, and then I sum the Total Billable Hours from the Timesheets table that have been performed, and then I subtract that sum from the Budget Hours.

"Projects"."Budget Hours"-sumif("Timesheets"."Status"='Billable',"Timesheets"."Hours",NULL)

Unfortunately, doing that gives me this error:

Mixing of Aggregate functions (like SUM, MAX, ...) with non aggregate columns is not allowed.

I've tried a number of different angles, but can't seem to find the right way to do this.

Cheers,

Andrew