Captivate 2019 - cannot open old .cptl file

A CPTL file is a Captivate template file, not a Captivate project file (CPTX).  For many years now we have been recommending Captivate developers NOT use CPTL files to spawn new projects because the template has a very nasty bug that Adobe has failed to rectify despite repeated requests.

However, 2018 is not that long ago.  The only two reasons that I know of for not being able to open a CPTL file with Captivate 2019 would either be that it was created with a much older version of Captivate that dates back many years or else the file itself is corrupted.

So, do you know what version of Captivate was last used to open it?

The template files are buggy since many versions, I am not astonished that you cannot create a project from an older cptl-file. Moreover, if you are on version 11.5 (CP2019 had many releases, two main ones) themes have been changed a lot and that could also add to the problem. You cannot convert a cptl-file to a cptx file, you need to create a project from it using the option File, New Project, Project from Template. No guarantee that it will work.