Ola Hallengren backup systax when excluding a database
it has to be 

@Databases =  'USER_DATABASES, -Db1'

See https://ola.hallengren.com/sql-server-backup.html

Assuming you want to backup all User Databases except one named DBA, your statement should be:

EXECUTE [dbo].[DatabaseBackup]
    @Databases = 'USER_DATABASES, -DBA',
    @Directory = N'G:\BACKUP',
    @BackupType = 'DIFF',
    @Verify = 'Y',
    @CleanupTime = 336,
    @CheckSum = 'Y',
    @LogToTable = 'Y'