Hi…,
No, you don’t need a Navigraph subscription to run MSFS flight planner.
I suggest reinstalling Navigraph Navdata Center as per instructions at Release notes - Navigraph Navdata Center
Use that to Remove and Install Navigraph Charts In-Game Panel.
Make sure you restart Windows after install.
If still an issue, please advise steps you are taking to “make or import an IFR flight plan”. Maybe upload the plan.
Cheers
Ian