After typing "c.reload Backpacks" into the console, the backpack shows up and everything works.
But it shows this error.

I'm using Carbon

19:46:19 c.reload Backpacks
19:46:19 Unloaded plugin Backpacks v3.13.0 by WhiteThunder
19:46:20 Loaded plugin Backpacks v3.13.0 by WhiteThunder [529ms]
19:46:20 Checksum validation failed for 'Network.Networkable.UpdateSubscriptions' [OnNetworkSubscriptionsUpdate]
19:46:20 Checksum validation failed for 'Network.Networkable.UpdateHighPrioritySubscriptions' [OnNetworkSubscriptionsUpdate [2]]

It sounds like you're describing two problems.

• Backpack not showing -- It's unclear to me what this means. Are you referring to the UI belt button, or are you saying that running the backpack command doesn't open the backpack?
• Checksum validation failed -- That sounds like a Carbon issue related to dynamic Harmony patching, not really anything the plugin can do about that.