You might need to revise the data in the column, but anyway you can do one of the following:-
1- check if it is numeric then convert it else put another value like 0
Select COLUMNA AS COLUMNA_s, CASE WHEN Isnumeric(COLUMNA) = 1
THEN CONVERT(DECIMAL(18,2),COLUMNA)
ELSE 0 END AS COLUMNA
2- select only numeric values from the column
SELECT COLUMNA AS COLUMNA_s ,CONVERT(DECIMAL(18,2),COLUMNA) AS COLUMNA
where Isnumeric(COLUMNA) = 1