Hi, I'm Bhavik. Welcome to JustAnswer. I'm reviewing your question now, and will reply back ASAP.
There is some ongoing issue with Skype in Sierra OS. Skype has not yet found the fix for this compatibility issue.
If you want, I can look for the workaround for it.
You can use web version of skype till the issue gets resolved. May be in skype's next update they will be able to resolve the issue with skype application. Till then you can use this link from your mac:-
Were you able to use it ?
Did you try to call to other numbers/users as well ?
I checked the compatibility of it. Everything seems to be okay. I would suggest you to use some other browser and see if you are able to make a call.