EOMONTH()
was introduced in SQL Server 2012.
I suspect the Compatibilty level of your database is set below that (or you are in fact not on SQL Server 2014).
ALTER DATABASE database_name
SET COMPATIBILITY_LEVEL = 120 -- 120 == SQL Server 2014
To determine your SQL Server version run:
SELECT @@VERSION
An alternative way of calculating EOMONTH is:
DATEADD(d, -1, DATEADD(m, DATEDIFF(m, 0, @Date) + 1, 0))