Seaquel
SQL Server error code

SQL Server error 1015

SQL Server raises error 1015 at severity 15, which marks a problem with the query or data that you can correct.

An aggregate cannot appear in an ON clause unless it is in a subquery contained in a HAVING clause or select list, and the column being aggregated is an outer reference.
Msg1015
Severity15
Group1000–1999

Source: Database Engine events and errors