It's hard to tell from the picture, but it looks like you're missing some critical parts. There's supposed to be a lock washer that holds the tube, and a plastic release for the tube so it can be removed. It looks like an Adventurer 3. If so you can order replacement parts from the manufacturer or here:
https://www.amazon.com/gp/product/B01NANKRTD/
You may also need a replacement tube if it's scraped up from multiple removals.
For what it's worth, I bought these and they worked perfectly!Witbot PC4-M6 Straight Fitting... https://www.amazon.com/dp/B01NANKRTD?ref=ppx_pop_mob_ap_share
You can just cut it back 1cm or so and try again...but I would also order a replacement pack of the fittings.
These are for the ADV3 but I think will for for you too: https://www.amazon.com/dp/B01NANKRTD?psc=1&ref=ppx_yo2ov_dt_b_product_details
While you are there you might want to just grab a capricorn tube for $10.
https://www.amazon.com/dp/B079P92HN9?psc=1&ref=ppx\_yo2ov\_dt\_b\_product\_details
This is what I ordered for mine adventure 3 last week and it worked great. https://www.amazon.com/dp/B01NANKRTD?ref=ppx_pop_mob_ap_share
Buy a new Bowden coupler. This is a common problem. I have a bunch i keep on hand.