Error Cannot find « » in scope XCODE

func checkValue(_string: String, value: String) -> Bool {
  let isNotEmotv = string != ""

There is an inconsistency between parameter declaration in func and the var used in the test.

You use the wrong parameter name in test. Or you have a missing space between _ and string. Which causes the compiler to look for a parameer named _string and not string.

First option, change the test on string to a test on _string:

func checkValue(_string: String, value: String) -> Bool {
  let isNotEmotv = _string != ""  // <<-- add _

or second option, change the parameter definition in the func declaration

func checkValue(_ string: String, value: String) -> Bool {  // <<-- add space
  let isNotEmotv = string != ""

Second option is preferrable, but then you need to change the calls as:

    checkValue(email, value: "adresse email"
    checkValue(password, value: "Mot de passe")

However, signIn is bizarre.

You call checkValue which is a Bool, not an optional. So guard does not make sense here.

 func signIn(email: String, password: String) {
    guard checkValue(_string: email, value: "adresse email") else { return }
    guard checkValue(_string: password, value: "Mot de passe") else { return }
  }

What do you want to do if check are not OK ?

Your code should probably be (if you select second option):

 func signIn(email: String, password: String) -> Bool {
  
    if checkValue(email, value: "adresse email") && checkValue(password, value: "Mot de passe") {
    // Do something when OK
    return true
   } else {
      // Not OK, alert user that if failed
      return false
   }
 }