I try to add a modifier and that goes all well. But even when it comes to that...I can't figure out how to replace it. I've tried but it said Error, need one or more modifiers (1 more) every time I try replacing a part on my tool.

You're trying.. to replace a modifier? That's not possible.

Update for 1.12.2; issue is still happening.

Config is set to 0 (so they start with 0 modifiers), and if a mod is added to a tool, then the tool can never have a part swapped out cause of the error message.

Same problem on my end. I ended up wasting the two other peoples time because I didn't think this could be causing it. I'm running the 1.0.3 version for 1.12
Seeing as the last post was 11 days ago an acknowledgement of the issue would be nice.

This actually is a TiC issue, which checks if modifiers are still valid when replacing a toolpart. However to do so it requires free modifiers (it just checks on the new tool, it doesn't actually consume the free modifiers). If it's set to 0, it always fails. I'll see to have this fixed in TiC.

I'm getting the same thing, but the original wording of the issue is a bit weird. What is being said is that when you swap parts without a modifier on the tool at all, you're fine. However, if a modifier is present on the tool and you try to swap parts, as shown in my image below, you get an error. I tested this with a new tool as well to throw out changing config mid play-through and it produces the same result. The tool provided has 2 modifiers gained from levels, one being used by the obsidian plate. I have not tested with any other modifiers applied to the tool.

Edit: I have the config set to 0.

Not sure if there is a mod conflict or not. Running both 1.10.2 versions of each mod.