I've never been able to, afaik that's just the way it is with a lot of barb/tubing combos - but once it's in your computer you really won't be able to see it.
If you're really committed to it, you'll need two pairs of liner lock pliers and two wash cloths (or one you rip in half) that you don't care about. Put a piece of wash cloth over both ends separately, grab each piece with a pair of liner-lock pliers and crank it down. There's no guarantee that you won't just snap the fitting, or what you'll actually make any headway.
Just remember that however much force you put into putting it together, you'll have to apply more than that to get it apart.