So, Steam is sending a code via SMS to your phone, but when you enter the code, Steam says the code is incorrect.
And even when you copy and paste the SMS code, Steam still does not accept it claiming the code is wrong. Well, authentication is all about correct date and time. So first thing you have to check is whether your phone is set to correct time zone. If not, either select correct time zone for your place or just enable automatic time zone selection. After that, set correct date and correct time or once again, use automatic settings. Reboot the phone and make sure timezone, date and time are correct. Now request another SMS code, enter it, and this time Steam should accept it.