Diagnosis
Canister latch jam. Debris has become packed into the latch mechanism or the plastic release lever has broken.
How to Fix It
Apply firm upward pressure on the bottom of the dirt canister with one hand. Simultaneously, press the red release button on the handle with your thumb. Use significant force. The design requires the canister to be slightly lifted while the button is depressed. If it disengages, clean all debris from the metal latch hooks on the canister and the corresponding slots on the vacuum body.
If That Didn't Work
The plastic linkage from the button to the latch may be disconnected or broken. This requires disassembly of the handle. Remove the screws on the handle's rear cover to inspect the internal lever and spring.
Heads Up
Forcing the canister without depressing the release button can break the plastic mounting posts on the main vacuum body. This is a major structural repair.