Why are you getting this error?
You are getting this error because no business network has been specified for the connection. This does not happen when you first time to run a composer playground because there is no network and you do not have a card too.
This happens when you stop/delete your business network, or there is no business network running but you also have the business card linked to the network.
What is the solution, then?
Follow the tutorial to create a new business network locally.
Restart your fabric tool by stopFabric.sh and startFabric.sh, (* this is not to start your business network but your fabric)
Run your composer-playground again.
When you see this error again, find a button at the top right corner. This will be folded as My Business Networks.
Delete the card that does not have a connection or simply deploy a new network.