@MaheshPadekar - The Activity entity is basically a pointer entity between the primary entity (Account, in your case) and the individual activity entities. There are separate entities for each activity: Appointment, Task, Email, Phone Call, etc.
You also referenced "post" in your message. There is a separate entity for Posts which can be referenced within Power Automate.
So, if your trigger is on Post creation, then you need to decide which of the Activity entities you are looking to update.
I want to update field on Account entity whenever a "Post" is created. I mistakenly mentioned "Activity" in my original post. I've now corrected it.
I have tried creating a flow that triggers on "Create" of "Post" entity record, but it does not have provide "Regarding" field which can be used to retrieve Account record. I also tried to retrieve Account records through "Post Regarding" entity but retrieve on "Post Regarding" entity is not supported.