Hi and welcome.

Can you upload a picture of your pickup wires? (pics need to be less than 1500 pixels on the longest side and under 1MB in size so you'll probably need to resize them, or else host elsewhere and provide a link).

The kit pickup wires change over time, so the diagrams are now often indicative, rather than fully factual in all details.

The pickups will either have
a) one central wire (hot/signal wire) with an outer braided screen (screen connects to ground/back of a pot) or
b) four coloured wires and an outer braided screen. You should find that one coloured wire should be soldered to the braided screen, two other wires are twisted and soldered together (and hopefully then insulated with some heat-shrink) and one single coloured wire on its own. This single wire is the hot/signal cable which goes to the relevant volume pot lug. The braided screen+coloured wire pair get grounded on a pot back, and the other pair remain unconnected.

Hopefully that still means you've wired the pickups correctly.

If so, it's possible you've got the output wired to the wrong connection on the barrel jack socket. If so, tbe signal from the pickups will get grounded when you plug a jack in. Unlike open jack sockets, they only seem to make barrel jack sockets in a TRS/'stereo' format, and the tip and ring connections aren't obvious as to which is which. It is also possible that you may have a loose strand of wire that is touching another of the tabs on the barrel socket and grounding the output. It doesn't take much.