hello all,
We are using ArcGIS Enterprise 9.1 and are trying to publish custom printing services from ArcPro 3.0.
When you publish* a print service from ArcGIS Pro 3.0, your Enterprise server must be 11.0 or above.
* more precisely when you create a layout template (aka pagx file) from Pro 3.0.
As you know ArcGIS Pro 3.0 is a major release. Document types such as projects, maps etc. that you create in 3.0 can't be directly opened in 2x. If you must share with older client, then you need share that out as project package, map package etc.
Since Enterprise 10.9.1 is an older version and it not aware of 3.0 changes, a print service running on 10.9.1 can't recognize layout templates created from 3.0 (even when you publish it from 3.0).
Please let me know if you have any question.