Sometimes you need to disable the Steam Guard Mobile Authenticator, which is an app generating one time passwords to for the 2-factor authentication at Steam.
The two most common cases are you have lost the phone, or deleted the Steam Guard Mobile Authenticator from the device. If you reinstall the app, it won’t work, as all the app data linking the particular installation to your Steam account was lost. You have four options to recover your account. The first one restore Steam Guard Authenticator app from a backup complete with application data, that you should have made beforehand with additional software like Titanium Backup. It usually requires root. Restoring such a complete backup will get you the Steam Guard Mobile Authenticator working on a new device. If you don’t have the backup, in the enter code page, tap on “Please help”, then “Remove”. If you have a phone number linked to your Steam account, and you still have access to the SIM card, Steam can send you a code that will remove the Steam Guard. That’s you second option. If you don’t have access the phone number anymore, click “I can’t” Here comes the third option. You should have the recovery R-Code written down when you were setting up Steam Guard. This code should not be entered in the main Steam Guard Mobile code window, where you normally punch in the OTP codes generated by the app. You should enter it in the following screen. However, using the Steam Client to deactivate Steam Guard with the R-Code often brings out errors like Sorry there was an error encountered while processing your request. And sometimes, the Authenticator recovery code provided is incorrect. Even if you enter a correct R-Code. To avoid the errors, simply open this link in your browser. When you open it in the browser, first search for your account, and then select Enter your Steam Guard Mobile Authenticator recovery code. On the next page, enter the R-Code, and the mobile authenticator will be disabled. That was the third option. If you don’t have the backup, or a linked phone number with the SIM-card, or the R-Code, then you have to set up a Steam Support account. Better register it on a different email, not the same you use for Steam. This is an additional security measure if someone steals your Steam account, he won’t get the Steam Support account. In the Steam support, open a ticket to remove the Steam Guard Mobile Authenticator. You have to include a proof that you are the owner of the account. This may be a bill or a screenshot of payment you sent to your steam account earlier, or an activation code for a game you have on your account. It takes 5 to 50 days for the Steam support to validate data provided, and remove the Steam Guard Authenticator.