Google account or samsung account?
It should only let you login to a real account that exists. Not exactly sure how you managed to login with an email and password, then not be able to enter the email and password. Not an issue though, there's a solution. Assuming your issue is with a samsung account, as I remember devices ask for that password during a reset.
As long as you know the login details to the google account linked to the device, you can reset it through the recovery menu. If no google account was ever linked, even easier. If the google account is the issue, don't reset it or things get worse.
To do a manual reset, power off the device.
Hold volume up and power till you see the samsung logo, then release. Some websites say connect it via a usb cable to a pc during this but I'm unsure of that.
You should now be in a new menu. Using the volume buttons to move and power to select, select reset and do as it says.
The device should now be reset. If you had a google account previously linked, login with thay google account. No other account will work, it has to be the original google account you used.
Mkk7781 actually linked a video on how to do this. Just do not reset if the google account is the wrong email one. That will brick the tablet unless samsung themselves unlock it
