I have tried publishing an app with the same FDK version. It goes through fine without this error. I couldn’t reproduce it.
I have checked with the Freshdesk product, and I do not have a classic Freshsales account. Do you mean to build this app only on the classic Freshsales product?
Can you try updating the FDK version to the latest and check if it works with the latest build?
If it still fails, we will have to check further with more access to your account. I can create a support ticket to check it over a private conversation.