Inserted a brand new cartridge, and opened the vent cap. The printer was stalled on waiting for resin to fill the tank.
I closed the vent cap, and removed the cartridge for inspection. The rubber seal at the bottom has not split because it had rotated out of alignment with the plastic piece the mechanism in the printer pushes it against to split it open.
Fix: With the vent cover closed, and the cartridge upside down, carefully re-aligned the rubber piece to be square with the cartridge, and manually pinched it causing the split to properly separate