Because that's not the correct syntax for the
Contains
method.
Try:
var subQuery = from c in t024
where c.c_causale.Contains("FIN")
|| c.c_causale.Contains("PUB")
|| c.c_causale.Contains("CHI")
select c.id_cau;
var query = from r in t011
where r.id_reg == 22880
&& !subQuery.Contains(r.id_cau)
select new
{
r.id_per
};