Quick Fix: Your account is low on credits. Add phone credits under Admin>Billing>Buy phone credits.
If an agent is not able to change their availability status it could mean that the account is running low on phone credits. If you are an account admin add phone credits under Admin>Billing>Buy phone credits.
If an agent is unable to set their availability status it could also be because they have not verified their account. In order to verify their account, they have to reset their password by going to their profile settings, they can then use the new password reset link to login. This will enable them to set their status as online.