Diagnosis
The quick-connect coupler on the hose end is not aligning or the O-ring is swollen/jammed.
How to Fix It
First, ensure there is NO pressure in the hose. Pull the trigger to bleed it. Look inside the female coupler on the hose. You'll see a plastic sleeve with tabs. Push that sleeve forward toward the gun. While holding it forward, insert the gun connector. It should slide in and click. Release the sleeve.
If That Didn't Work
The O-ring inside the coupler might be damaged or missing. Remove the coupler from the hose (wrench on the brass fitting). Replace the O-ring. Use a petroleum-resistant one.
Heads Up
Never force it. Cross-threading the brass fittings will ruin them. The connection must click and lock. Test by pulling back on the hose—it should not disconnect.