There can be two reasons why an agent can't switch their availability status:
The account might be running on Low Credits:
To check and add credits to your account, contact your admin. The following are the steps to be followed to add phone credits:
Go to Admin Settings > Account Settings > Plans and Billing > Phone Credits.
Select the amount you want to add and click continue. For details, see How to add phone credits?
The agent has not verified the account
If you have not verified your account after signing up, you will not be able to change your availability status.
So, ensure that your account is verified. To do this, reset your password under profile settings. Use the new password reset link to login.