Soldering XT60 connector
The tip of your soldering iron looks like it doesn't &quot;wet&quot; well, i.e. it solder doesn't adhere to it, which means there's little heat transfer between the iron and the solder, and soldering won't work.
Soldering thick wires/connectors requires higher powered soldering irons, with a good tip.
Considering the red tape probably also hides a defect, this iron has a screw that doesn't even fit and and the tip looks abused:
Buy a new soldering iron that's less terrible. Doesn't need to be great. This works reasonably well for the occasional solderer.