A seating chart I made cannot be edited. What gives?

Floor plans must be owned by an organization. When a Partner or Portal user creates a floor plan it is made without an organization and will become stuck in the system as they are.

The solution is to "scope into" the organization that should own the floor plan you want to create. Do this by clicking on Organizations and finding and clicking on the relevant organization. Then, head to the relevant venue and make the floor plan. Now the floor plan will be owned by that organization and there is the option to edit that floor plan IF you are scoped into the organization that owns it.

More info about navigating the back end as a Partner/Portal user is here.

More info here about making floor plans.