There can be two reasons if the agent is not able to change their availability status:
Account Running on Low Credits
To add credits to your account, contact your admin.
If you are the admin of your account, recharge your phone credits.
To do this, 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?
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.