Version Info

• Minecraft, ...1.21.1
• Packed Up, ...1.0.30+b Forge
• Core lib, ...1.1.17

Steps to Reproduce
Not sure what this means?
...
Crash report (~/logs/latest.log)

Internal Exception: io.netty.handler.codec.Decoder.Exception: Failed to decode packet 'serverbound/minecraft:set_creative_mode_slot'
...
Basically me and my gf wanted to play with some quality of life mods. We have a few installed such as:
Macaw's Bridges
GlitchCore
SuperMartijn642's Config Library (Version 1.1.8(yours ofcourse)
SuperMartijn642's Core Lib
(Version 1.1.17)
Packed Up
Storage Delight
Just Enough Items
MonoLib
Common Networking
Golden Foods
Forge
Sound Physics Remastered
Journey Map API
Journey Map
Nature's Compass
All of these are using forge.

The mod works in single player from the crafting to the using of the item but on the realm the recipe wouldn't show up in the crafting table and then I tried using creative and giving myself the item to see if that worked and that's when I get hit with the crash report.

I'm new to mods so have no idea how to fix anything myself. Any help, thanks.

Can you share the logs?

Actually, now that I look at this again, you mention playing on realms. Packed Up needs to be installed on both the server and the client.
On Realms, you do not have access to the server, thus you cannot install mods on it.