When creating an entity through the API, the API user may reference a template eid that hasn't been shared with that user or their group.
This will return the following error:
"title": "The resource your required is not found."
You can confirm if the template is shared or not by issuing the following API request:
If the template has not been shared go to:
SNCONFIG->System Objects-> Find the template in question->Click on the template and then click on Sharing on the top right.
Make sure it is shared directly with the API user or through a System Group.