To successfully log in using two-factor authentication, you must enter your password and a one-time confirmation code. This confirmation code is generated by your authentication app.
To ensure security, the FreeOTP authentication app generates a confirmation code that expires after a certain amount of time to ensure security. If you are unable to successfully log in with the generated confirmation code, the code may have already expired. The FreeOTP app automatically generates a new code that you can use if you were not able to enter the previously displayed code on time.
If you should encounter a similar problem when using another authentication app, you can also visit the service provider's help pages for assistance: Selection of apps for two-factor authentication