Steam Mobile Authenticator not working?

Solution 1:

There is no way to get around this limitation. After you wait 7 days, you will no longer have trade holds.

The most likely cause of the sudden trade hold is because of a disable and re-enable of the mobile authenticator or multiple failed attempts at entering the authentication code.

Solution 2:

Have you tried to remove the authenticator and re-add it?

• Disable the authenticator (go to the Steam Guard menu and choose a different Steam Guard method)
• Log out of your account (go to the Log Out menu on iOS, or go to Settings / Log Out on the Android app).

Source