Easy fix, Get Hercules 95-5 in paste form.
Try to clean the joint as much as possible inside and out.
Use this Solder/paste as a FLUX
Use a turbo torch flame (4,200 degrees) secondary heat to heat the pipe then the fitting.
As this “flux” turns silver you can add more 95-5 tin antimony solder (Spool)then take away the flame and let Capillary action draw the solder into the joint.