Working on evalExpr :: can't fix an error?

i’m working on a Evalexpr in haskell and i get this error :
"Unexpected do block in function application:
do (v1, cs1) <- digitParser cs
You could write it with parentheses
Or perhaps you meant to enable BlockArguments?"

my function :

opperaParser :: String -> Either SyntaxError (Eval, String)
opperaParser cs =
    do (v1, cs1) <- digitParser cs
    case cs1 of
        [] -> return (v1, [])
        _  -> do
            (plus, cs2) <- findOpperators cs1
            (v2, cs3)  <- opperaParser cs2
            return (plus v1 v2, cs3)

opperaParser :: String -> Either SyntaxError (Eval, String)
opperaParser cs = do  -- ← indent lines under the do
    (v1, cs1) <- digitParser cs
    case cs1 of
        [] -> return (v1, [])
        _  -> do
            (plus, cs2) <- findOpperators cs1
            (v2, cs3)  <- opperaParser cs2
            return (plus v1 v2, cs3)