If you want to copy data from table b on schema1 to schema2, you need to connect to schema2 and grand select permission first:

grant select on b to schema1;

Then you can connect to schema2 and copy data using “insert into schema2.b select * from schema1.b”.
You may still get error, then list the column names:
insert into schema2.b(col1, col2) select col1, col2 from schema1.b