Order of your If statement is not correct. you should first check for null values in both A &B Columns. And in the next "if-else" statement, you should check if A is null and B is not null then column A else column A
Structure of IF-ELSE Statement:
IF A= Null() and B= Null() then "N/A"
elseif A=Null() and B!=Null() then [B]
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