iPhone forgets Apple ID password
Why does my iPhone suddenly decide it needs me enter my Apple ID password? This is inconvenient, in a trip without all my other trusted devices.
There are a number of reasons that you are asked to re-enter your Apple ID password. If you don't have a trusted device handy, there are also other methods to gain access to your ID's verification code.
Ref: Get a verification code and sign in with two-factor authentication - Apple Support