That error message doesn't tell exactly the origin of the problems, which is a pity.
There have been two main releases under CP2019 and plenty of small releases. Can you please check the exact version number you are using? You find it under Help, About Captivate.
The main releases (11 and 11.5) are not compatible: you are not able to open a project created with 11.5 in one of the 11.0 releases. First thing to check. Most recent cptx files in the community will be made with 11.5, if you are still on 11.0 that could explain the error.