Plane not allowed on flight but others are using it

I checked market analysis etc and bought 2 planes. Set up cabin and such and when i tried to create flight plan i found the plane is not allowed on route.

The reason i picked this plane is because other airlines are using it on the same routes i chose.

Eventually, they'll no longer be allowed to operate that aircraft either.  We're in a grace period to permit existing players time to change their schedules and fleets to accommodate the new regulations.  Pre-existing flights from before the change are allowed to continue to operate, but no restricted aircraft can be scheduled or re-scheduled.