A couple days ago I was working on some EDI case with the Logic Apps. As you already know you need a "Trading Partner Management" API App to use AS2, EDIFACT, ...

So I created a Trading Partner Management API App.
Then I tried to add a Partner in the "Partners Component".

I received the following error:

Together with the great team behind the Logic Apps we found what was wrong.
They asked me to check the DbConnectionString in the API Host.

Part of the Appliction Settings

As you can see the DbConnectionString is empty. That's definitely not good.
Go to your Trading Partner Management database and select a connectionstring.

You can do that like in the screenshot below , by clicking on the "Show database connection strings" link.
Just select 1 connectionstring it doesn't matter which one. Copy that one and change the username & password.

Now paste the connectionstring into the empty DbConnectionString on your Trading Partner Management API App Host (Application Settings).
I restarted the API Host and tried directly to create a new Partner. But that failed again. Just wait 5 minutes and then it will work. :)

Again thank you to the fast response from the Logic Apps Team. I hope this will help people with the same problem.

Any question or remark. Post it below....