The cause could be that the API access has been disabled by policy. Check the folling steps below, which would resolve this issue.

Using Windows Registry Editor (regedit) open this key:

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Skype\Phone

Can you see in the right hand panel an item with this name: “DisableApi”?

If the value in the Data column is 1, then this means that API is disabled for all users on this machine. In order to enable this option, you can change the value to 0 or just delete the key.

If the Policy settings are applied only to the current user, then a similar item can be set in this registry key:

HKEY_CURRENT_USER\SOFTWARE\Policies\Skype\Phone