I made a modded server with my friends (fabric). I can't charge the jetpacks or I just don't know how. If you could help me that would be appreciated thanks!

From my experience, there's currently no way to charge it without another mod. If you have Industrial Revolution, you can craft a Lazuli Flux Container (Battery which requires a infuser, compressor and a pulveriser to make) that can charge the jetpack, or you can use a charging pad (Requires more resources.).