Select from multiple table MS SQL

Question

What I wrong ?

 

select x1.poslovni_partner as 'Poslovni partner',
CONVERT(decimal(16,2),x1.Nabavka) AS 'Prijem',
CONVERT(decimal(16,2),Izdavanje,0) AS 'Izdavanje'
from(select poslovni_partner, iznos_sa_porezom as Nabavka
from (select poslovni_partner, SUM(ISNULL(iznos_sa_porezom,0)) as iznos_sa_porezom from mp_ulazni_racun_roba_lista union all select poslovni_partner, SUM(ISNULL(iznos_sa_porezom,0)) as iznos_sa_porezom from mp_ulazni_racun_ostalo_lista) a
group by poslovni_partner, iznos_sa_porezom) x1
left join(select poslovni_partner, iznos_sa_pdv as Izdavanje
from(select kupac as poslovni_partner, SUM(ISNULL(iznos_sa_pdv,0)) as iznos_sa_pdv from mp_racun_lista union all select kupac as poslovni_partner, SUM(ISNULL(iznos_sa_pdv,0)) as iznos_sa_pdv from mp_faktura_lista union all select kupac as poslovni_partner, SUM(ISNULL(iznos_sa_pdv,0)) as iznos_sa_pdv from vp_faktura_lista) b
group by poslovni_partner,iznos_sa_pdv) x2
on x1.poslovni_partner = x2.poslovni_partner

Error is 

 

Msg 8120, Level 16, State 1, Line 30
Column 'mp_ulazni_racun_roba_lista.poslovni_partner' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.