Apple ID verification failed

See Get a verification code and sign in with two-factor authentication https://support.apple.com/en-us/HT204974  Below is an excerpt, but read the whole document. See the bold italics below.

Whenever you sign in with your Apple ID on a new device or browser, you'll confirm your identity with your password plus a six-digit verification code. There are a few ways you can get a verification code. You can use the code displayed on your trusted device, get a text or phone call, or generate a code from your trusted device [see the additional information in the article]. 

A trusted device is an iPhone, iPad, or iPod Touch with iOS 9 and later or a Mac with OS X El Capitan and later that you've already signed into using two-factor authentication.

If you can’t sign in, reset your password, or receive verification codes, you can request account recovery to regain access to your account. This link takes you to:

Recover your Apple ID when you can’t reset your password  https://support.apple.com/en-us/HT204921 which you may need to use.

“Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. After you request account  recovery, you'll get an email with a confirmation of your request and the date and time of when you can expect to regain access. … When the wait period is over, Apple sends you a text or automated phone call with instructions to regain access to your account.” Read the whole document carefully. 

I am currently logged in using my Apple ID and password so I can reply to your post, yet my single solitary apple device, an iphone 6, will not let me update whatsapp saying "verification failed" due to username/password being wrong.... Now how can this happen? Your answers above don't address this situation. My login works when I use my computer, but not to update an app. Why? the phone is listed on my account (it's my only device) and it says I'm currently using it.

Some other things you can try. Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

If necessary

Hard reset.        

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Backup - Restore your device from an iCloud or iTunes backup.      

Device or computer? Do you have access to another WiFi network where you can try? That has worked for several people.