How to fix this PTFE tube?
I had the same problem. I ordered new Tubes from PRUSA. The metal ring is normaly pressed on the tube. So you can't make that work again. It would be better ich you could use normale PTFE Tube with other connections.
The Brass Compression Fitting are not made to be reused. As a mechanic background once the compression fitting falls off or crack or not compresses on tube improperly, on metal plastic rubber it’s no good. Two things you can do buy a replacement at the prusa Eshop or Go to local hardware store see if you can find one that’s close and that will seal around your Bowden tube and fit the brass fitting that goes into nozzle assembly. You need to make sure you cut off the part of the Bowden tube That has been compressed already otherwise it would never seal right. If you have some new bowden tube the same size I would just replace the whole thing with it two new compression fittings that work with the fitting that goes into the nozzle assembly ensure it’s pressed completely down to where it stops and it will be at the end of the tube so it doesn’t fall off the end again. I would used two wrenches the whole the brass fitting will you tighten down both ends to compress the ring around the Bowden tube.