That is correct, I did miss a closing parentheses.
But the program still won’t run in GHCi
and the error now is saying the following:
GHCi, version 9.4.8: https://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling MonadicParsingInHaskell ( MonadicParsingInHaskell.hs, interpreted )
MonadicParsingInHaskell.hs:12:10: error:
• No instance for (Applicative Parser)
arising from the superclasses of an instance declaration
• In the instance declaration for ‘Monad Parser’

12  instance Monad Parser where
 ^^^^^^^^^^^^
MonadicParsingInHaskell.hs:45:40: error:
• No instance for (MonadPlus Parser) arising from a use of ‘++’
• In the first argument of ‘parse’, namely ‘(p ++ q)’
In the expression: parse (p ++ q) cs
In the expression:
case parse (p ++ q) cs of
[] > []
(x : xs) > [x]

45  p +++ q = Parser (\cs > case parse (p ++ q) cs of
 ^^
Failed, no modules loaded.
I think the error is coming from this instance:
instance Monad Parser where
return :: a > Parser a
return a = Parser (\cs > [(a,cs)])
(>>=) :: Parser a > (a > Parser b) > Parser b
p >>= f = Parser (\cs > concat [parse (f a) cs'  (a,cs') < parse p cs])
In particular I think it might be this line
"No instance for (Applicative Parser)" that is causing the errors. Though I don't exactly know what it means.