Hi,
It looks like the custom posting (aka Activity Feeds) is not available in PowerApps, only in CE/CRM apps.
The error is because your Power Automate Block doesn't contain the pointer to the record where the post should be posted. See how this should look in Sales Environment:
In PowerApp-only environment the "Regarding" fields are missing, compare:
However, (in CRM!) you can even set your own custom tables to be available for this feature. To do this, you have to navigate to Advanced Settings > Activity Feeds configuration:
I described here, how to enable posts on custom entities.
The bad news is it seems like this feature is not available on enviroments without Dynamics Apps. There is no "Activity Feed Configuration" option for PowerApps.
But don't worry - you still can create your own custom activity table and use is as post, or you can create a Note to put some input on your timeline.
