I ran into the very same issue. Upgraded from ECE 2.1 to 2.2, attempted to create a new ccs cluster using Cross cluster search template, customized the deployment and the error above showed up.
The root cause is that the deployment template requires an instance template called "ccs.default" which doesn't exist. I fixed that by updating the template and using "data.default" instance template instead.
You can update the template easily by api call https://www.elastic.co/guide/en/cloud-enterprise/current/Templates_-_Deployments.html#set-deployment-template or simply creating an instance template called "ccs.default" might work as well, but I haven't tried.