I have tried giving permissions but it says permissions backpacks.use doesn't exist. Backpacks.admin doesn't exist. When I type plugins it shows backpacks on there- Backpacks additional text encountered after finished reading JSON content: o. path '', line 4, position 0.

That json error means the plugin failed to load. There is probably an issue with the main data file at oxide/data/Backpacks.json. Try deleting it and reloading the plugin.

I tried deleting and reloading the plugin. I even tried deleting the download to redownload it. Same issue every time. I downloaded other plugins just to see if they all would have issues but they work. 

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This is what is in the JSON file. Not sure if something here helps.
{
"Automatic Updates": {
"Enabled": true
},
"Spawn Handlers": {
"Enable loot container spawn handlers": true,
"Enable resource spawn handlers": true,
"Enable NPC spawn handlers": true,
"Enable APC spawn handlers": true
},
"Respawn Times": {
"Default loot containers": {
"Minimum (minutes)": 30,
"Maximum (minutes)": 60
},
"Desk keycard": {
"Minimum (minutes)": 15,
"Maximum (minutes)": 20
},
"Diesel Collectable": {
"Minimum (minutes)": 30,
"Maximum (minutes)": 45
},
"Junk piles": {
"Minimum (minutes)": 20,
"Maximum (minutes)": 45
},
"Resources": {
"Minimum (minutes)": 20,
"Maximum (minutes)": 45
},
"Traps/Barricades (Respawn/Re-Arm)": {
"Minimum (minutes)": 25,
"Maximum (minutes)": 40
},
"Vehicles": {
"Minimum (minutes)": 45,
"Maximum (minutes)": 60
}
},
"Version": {
"Major": 1,
"Minor": 0,
"Patch": 42
}
}

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I also found this in the oxide/data for backpacks.
{
"PlayerGuiPreferences": {}
}
o.grant group default backpacks.gui

There is a red X before number 4 which is the 0.grant group default backpacks.gui. When I hover over the red x is says Syntax error

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I figured it out. You mentioning the oxide/data/backpacks. When I seen that syntax error. I decided to delete that line and it now works. No clue how it got there but it was. Thank you for pointing out where to find the issue.