Admin user is unable to login
Please check if you are using the correct username (you can see that
in the database).
Try to use the "Forgotten password" function to reset your password
Check the last entries of the watchdog table to see if you can get
more information from there
Check your server logs to see if you get any error there (under Linux
this would normally be somewhere under /var/logs).
Install drush and use that to very easily log in
(https://drushcommands.com/drush-7x/user/user-login/)