Unable to sign in

Where are you trying to sign in? What happens when you try? Error message(s)? More information would help.

If you are trying to sign into iCloud, some people have solved this by going to System Preferences/Internet Accounts, selecting an iCloud account there, and signing in. You need to repeat this for each iCloud account.

Please see these Apple documents.

If you forgot your Apple ID password  

https://support.apple.com/en-us/HT201487

If you forgot the answers to your security questions

https://support.apple.com/en-us/HT201485 

https://support.apple.com/en-gb/HT204169

If your Apple ID is locked or disabled

https://support.apple.com/en-us/HT204106

If your device has been disabled in the App Store and iTunes

https://support.apple.com/en-us/HT208856

You can also contact Apple ID Support for help

https://support.apple.com/apple-id