External verification required?

I got a mail.
it says "Your item is rejected
"APP_NAME" is rejected for the following reason:

External verification required to make the app work.

What do you mean by that?

Please help me.

I think it means your app is not ready to use , it need some verification process after the purchase which is not legal to sell here on the marketplace.
All the app or script is ready to use where user purchase and work without any external verification from buyers like purchase code or anything else.
Thanks !

Got It.

I am using third-party APIs in my application and buyers need to set up an API key to make it work.
Is this a problem?