Error in SQL statement: AnalysisException: cannot resolve '`T_B.N`' given input columns

Question

I need a help. The error "Error in SQL statement: AnalysisException: cannot resolve 'T_B.N' given input columns: []; line 3 pos 10;" comes out when run the code. How i fix that? Is there an better way to write the query?

My "colleagues" and me have tried to solve in the ways we know, but it hasn't been done anyway.

Thanks for the attention

UPDATE T_A
SET dimRg =  (CASE
                    WHEN T_B.N < 10000 THEN "Small"
                    WHEN T_B.N > 29000 THEN "Large"
                    ELSE "Medium"
                   END)
WHERE T_A.Rg = T_B.Rg;