0. ALWAYS PROVIDE INFORMATION ABOUT MODLOADER (Fabric / Forge) AND MOD VERSION (For Example 1.19.2-8.2.10)

I've installed the 1.21.1 version for fabric on my minecraft modpack that I've put together on curseforge ( modpack code here : iF9J0wvH )

1. Describe the bug

The upgrade doesn't work. Even though I have a diamond backp it only appears with the number of slots for a leather backpack (9*3)

2. Write steps to reproduce the bug

I've just downloaded the mod and played with it

3. What is expected behaviour

4. Screenshots (If applicable)

Diamond Backpack is a skin for backpack, to increase the storage slot count you have to apply Tier Upgrades in smithing table to your backpack

Oh thank you, do you need a certain template to do it ?

There should be leather in template slot, and tier upgrade in addition slot:

https://github.com/Tiviacz1337/Travelers-Backpack/wiki/Crafting-Recipes

See here