Trouble with ID 20.

If your designer keeps both versions of InDesign installed, make sure your designer does not double-click on files to open InDesign files. It will usually go to the last installed version, which may or may not be 19. The designer needs to open the appropriate version of InDesign and use the Open menu.

If you are staying on 19, make sure both of you adjust your Creative Cloud preferences so you don't remove old versions when updating. 

@davidh24280568

You can always install v. 20 / 2025 together with v. 19 / 2024.

Just make sure you DON'T select option "remove old version". 

You can have multiple MAJOR versions installed at the same time. 

You can even open them together, at the same time. 

what's your operating system version?

I suppose it's possible to ask your designer using 20 to always export IDML for you instead of sending INDD, as IDML is the export format that one uses when you need to save back to an earlier version. But as a standard part of your workflow, I'd advise against relying on it. Better to get everyone on the same version. 

You and your designer really ought to be using the same version of InDesign. Some kinds of tools make it easy for one person to use Version X and someone else to collaborate using Version X+1, but that's not the way it works in InDesign (or any other page layout app, to my knowledge). 

Is your designer really "yours?" If so, tell 'em to keep 19 installed, and to use it on all of your work. It's not hard to install v19 from the Creative Cloud app. There's a "Show Older Apps" selector in the Apps section of the Creative Cloud preference.