You may try to check again the instructions from the link (https://developers.google.com/android/guides/client-auth), or you may check the video (https://www.youtube.com/watch?v=W3Zno0j4kiU&list=PLxefhmF0pcPmTIE0yl7wNw7JqUceYuL_L&index=3&pp=iAQB) which might also help you to check if you get a SHA correctly, or if you missed out something.

If both ways do not help, you may refer to the link (Flutter and google_sign_in plugin: PlatformException(sign_in_failed, com.google.android.gms.common.api.ApiException: 10: , null)) and find for more answer.