you're repeating the keyword
case
The following is how you use
case
SELECT CASE WHEN ColumnA = 1 THEN 'A' WHEN ColumnA = 2 THEN 'B' ELSE 'C' END
Although if you're only checking a single value and not multiple expressions the following is also valid.
SELECT CASE ColumnA WHEN 1 THEN 'A' WHEN 2 THEN 'B' ELSE 'C' END