I'm unable to place backpack in the trinket slot.

Mods:
[✔️] advancementinfo-1.18.2-fabric0.47.10-1.3.1
[✔️] AdvancementPlaques-1.18-fabric-1.4.4
[✔️] animal_feeding_trough-1.0.3+1.18.2
[✔️] appleskin-fabric-mc1.18.1-2.3.0
[✔️] architectury-4.1.36
[✔️] autoconfig1u-3.4.0
[✔️] BasicShields-1.3.0-1.18.2
[✔️] bettermounthud-1.1.4
[✔️] BetterPvP_22.4.0_Fabric_1.18.2
[✔️] BiomeWeatherEffects-1.3.3
[✔️] borderless-mining-1.1.1+1.18
[✔️] brb-Fabric-1.5.6
[✔️] cammies-minecart-tweaks-1.4
[✔️] carrier-1.10.0
[✔️] cem-0.7.1
[✔️] chatsounds-1.1.2
[✔️] CITResewn-1.0.1+1.18.2
[✔️] clickthrough-1.18.2-fabric0.47.10-0.4
[✔️] cloth-api-3.2.64
[✔️] cloth-config-6.2.57-fabric
[✔️] colormatic-3.1.1+mc.1.18.2
[✔️] combattweaks-1.11
[✔️] completeconfig-lib-1.3.3
[✔️] continuity-1.1.0+1.18.2
[✔️] Couplings-1.7.1+1.18
[✔️] CraftPresence-Fabric-1.18.2-Release-1.8.8
[✔️] cullleaves-2.3.2
[✔️] deathlog-0.2.7+1.18
[✔️] deepslatecutting-1.18.2-1.3.2
[✔️] DisableCustomWorldsAdvice-2.0
[✔️] DripSounds-1.18-0.3.0
[✔️] dynamic-fps-2.1.0
[✔️] EffectsLeft-Fabric-1.18.1-0.1.0
[✔️] enhanced_attack_indicator-1.0.2+1.18
[✔️] enhancedblockentities-0.6+1.18.2
[✔️] entity-texture-features-1.18+_2.5.16
[✔️] environmentalcreepers-fabric-1.18.0-1.6.0
[✔️] expandedstorage-7.3.11+1.18.2
[✔️] extrasounds-2.2.1+1.18.2
[✔️] Fabric-1.18-SmoothSwapping-0.4.4
[✔️] fabric-api-0.48.0+1.18.2
[✔️] fabric-language-kotlin-1.7.1+kotlin.1.6.10
[✔️] FabricShieldLib-1.5.3-1.18
[✔️] FallingTree-1.18.2-3.4.1
[✔️] feature_nbt_deadlock_be_gone_fabric-2.0.0+1.18.2-rc1
[✔️] ferritecore-4.2.0-fabric
[✔️] firstperson-fabric-2.1.0-mc1.18-pre1
[✔️] ForgeConfigAPIPort-v3.2.0-1.18.2-Fabric
[✔️] Gamma-Utils-1.5.6-mc1.18.2
[✔️] grasskiss-1.18.2-fabric0.47.10-0.5.0
[✔️] HorseBuff-1.18-1.2.0
[✔️] Iceberg-1.18.2-fabric-1.0.38
[✔️] indium-1.0.2+mc1.18.2
[✔️] inmis-2.5.2-1.18.2
[✔️] inspecio-1.3.0+1.18
[✔️] interactic-0.1.10+1.18
[✔️] InventoryProfilesNext-fabric-1.18.2-1.3.5
[✔️] iris-mc1.18.2-1.2.2-build.32
[✔️] item-model-fix-1.0.3+1.18
[✔️] kibe-1.9.7-BETA+1.18
[✔️] krypton-0.1.9
[✔️] lambdabettergrass-1.2.3+1.18
[✔️] lambdynamiclights-2.1.0+1.17
[✔️] language-reload-1.2.1
[✔️] lazydfu-0.1.2
[✔️] leaves-us-in-peace-1.0.4+MC1.18-1.18.2
[✔️] lithium-fabric-mc1.18.2-0.7.9
[✔️] midnightlib-0.4.0
[✔️] modmenu-3.1.0
[✔️] ninjaphenix-container-library-1.3.0+1.18-fabric
[✔️] notenoughanimations-fabric-1.4.0-mc1.18-pre1
[✔️] notenoughcrashes-4.1.4+1.18.2-fabric
[✔️] owo-lib-0.6.0+1.18
[✔️] Patchouli-1.18.2-66-FABRIC
[✔️] polymorph-fabric-0.0.19-1.18.1
[✔️] puzzle-1.2.1
[✔️] raised-1.18.2-1.0.4
[✔️] reeses_sodium_options-mc1.18.2-1.4.2
[✔️] respawnablepets-1.18-3
[✔️] RoughlyEnoughItems-8.0.442
[✔️] shulkerboxtooltip-3.0.7+1.18.2
[✔️] sodium-extra-mc1.18.2-0.4.2
[✔️] sodium-fabric-mc1.18.2-0.4.1+build.15
[✔️] soundphysics-fabric-1.18.2-1.0.4
[✔️] spelunker-1.2.0
[✔️] stacker-2.1.3
[✔️] starlight-1.0.2+fabric.89b8d9f
[✔️] trinkets-3.3.0
[✔️] veinmining-fabric-0.0.12-1.18.2
[✔️] visualoverhaul-4.1.0
[✔️] wthit-fabric-4.8.0
[✔️] XaerosWorldMap_1.20.5_Fabric_1.18.2
[✔️] youre-in-grave-danger-1.3.1
[✔️] Zoomify-1.4.0

I can use the trinket back slot with version 1.18.2.
Except in Creative Mode.

This means that any mod you are using is expected to conflict with inmis's backslot.
It might be helpful to remove the mods one by one and check what the problem is.
Don't forget to backup before erasing the mod.

I'll see if I can find the mod, it does work without any mods after all.

Turns out it was Inpecio that was causing trinkets to fail entirely. After deleting Inspecio, multiple trinkets began to work, including Inmis's backslot.