Stored procedure selecting from another scheme

When I try to run this stored procedure I get this error

PLS-00201: identifier schema2.table3 must be declared


I have looked everywhere and can't find the answer to this. I come from
a T/SQL background so I am at a loss as to why this doesn't work.

CREATE OR REPLACE procedure schema1.select_ssstudent
IS
begin
select a.*,
b.*,
c.sitename
from schema1.table1 a,
schema1.table2 b,
schema2.table3 c
where a.permnum=b.permnum
and substr(c.siteid,2,3) = trim(a.schoolnum)
order by a.schoolnum, UPPER(a.lastname), UPPER(a.firstname);
end;

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