Hello, I would like to create a modpack with MCA and I noticed that we could not modify the tags of the items.

Sorry for the late reply, what do you mean by that? Are you referring to tags, e.g. #wood or tags in terms of nbt tags, e.g. description/lore, title of an item?

This is not mca related, tags are always possible. What specific issue do you have?

When I try to change the tags in the modpack configs but it doesn't work.

Yes I am referring to tags like #wood.

Define doesn't work, what tag, what did you wanted to achieve and how. What confuses me the most is that you speak about config. Tags are defined in datapacks, not configs.