Already added your phone number and can’t login? here is a method created by Brady to solve this problem.
Note: this method works for the current Open Beta version. we will update it as the full version is released.
How to Fix Error Linking Phone
Can’t Login
Run into this error after already adding and verifying your phone number?
FIX
Go to Steam’s settings in the top left of the client.
Click on “Web Browser” and then click “Delete Web Browser Data”