If the template has content links that contain image layers, all the image layers must contain a file.
The most common reason a template will not open for a System Administrator in the Admin area is that an image is missing from the default content item assigned to the content link. Populating the field with an asset will allow the template to open again.
If you do not know which content item is missing an asset, you can reach out to the Pica9 team for assistance in diagnosing the issue.
Note that this issue is different from an End User trying to open a template on the front end. If an End User is unable to open a template, it usually means that they do not have access to the default content item for one of the layers. granting the End User access to the content item will allow the template to open.