(from 2k's site)
If you've encountered the error message shown here and are repeatedly being prompted to enter your 2K-associated email to no avail, we have some quick tips to help get you back on the courts!
If you’ve launched NBA 2K18 only to find that your account has expired, don't panic. First off, please verify your account.
If you aren't sure which email address is linked to your 2K account, please verify this in-game from the Main Menu > Options/Features > Options > Settings> scroll all the way down to 2K Account - Email Address.
Verifying your account is an easy way to solve a lot of gameplay and connectivity issues. If you don't see the verification email in your inbox, check your Junk Mail folder, as it may have ended up in there.
If none of this works, try manually changing the email connected to your account. This should send out a new confirmation email and allow you to verify your account.
To do this, head back over to the Main Menu > Features/Options > Settings> 2K Account – Email Address and change the email connected to your account.
From there, follow the verification link in the email and you should be back in online!
NOTE: When filling out your 2K-associated email for the game, make sure to enter a valid email format (eg. email@example.com). This is important: If you don't enter a properly formatted email, a verification email will have no way to make its way to you and will result in an expired account.