2FA is quite forward and should always work, however there are times where the issue by might locally from your device and here are the most common issues for 2FA code not being accepted in your account:
Wrong date or time on you mobile or PC:
It is always important to keep the time and date on your device up to date, wrong time or date may lead to 2FA codes being generated out of date and thus they may not work.
In order to avoid this issue check the time and date on your devices and make sure they are up to date and correct.
Time sync for 2FA app:
Some apps such as Google Authenticator may fall out of sync, thus giving you wrong codes that doesn't work, this issue can we fixed by running a time correction for codes inside the google authenticator app, this is how you do it:
Open "Google Authenticator" app
Click on the "3 dots" in the top right corner
Click on "Settings"
Click on "Time correction for codes"
Click on "Sync now"
And here you shall receive a message of "Time already correctly" or "Time synched" and now try using the generated code on your account again.
Wrong account:
In this case you might be using the code of an another account, make sure the code you are using is for the account you are trying to login to, if you aren't sure which account the code is for you can always use your secret key and recover the 2FA on your app.