I might be missing something here but I'm not sure you need those sub-queries there as you're alread joining on the table fo. See if this works:

select CREND = PL.CREND, NOME = PO.NOME, NCONT = PO.NCONT, EVIRS = sum(PL.EVIRS), EREC = sum(PL.EREC), U_TXIRS = isnull(fo.txirs,0)

, U_VLIRS = sum(case when isnull(fo.txirs,0) > 0 then PL.evirs/ isnull(fo.txirs,0) * 100 else 0 end)

from pl (nolock)

inner join po (nolock) on pl.postamp=po.postamp inner join fo on pl.fcstamp=fo.fostamp

where convert(char(10),PL.RDATA,121) BETWEEN '2007-01-01' AND '2007-12-31'

AND PL.EVIRS > 0

group by pl.crend,po.nome,po.ncont,pl.fcstamp