Existing tools cannot level

[Shelliac]

This may be working as intended, or may be a potential update for future.

When adding the Tool Leveling to an existing modpack, any Tinker's tools made before leveling was added cannot be leveled up unless you can modify them. So, any tool with an available modifier slot can be leveled up after another modifier is added, but if there are no slots free, the tools will never gain XP and level up.

[Dddsasul]

Same problem but considering the mod was last updated in 2018 I highly doubt this will be fixed
In the meantime a workaround would be to manually edit the nbt tag of the player, it should look something like in this image (note that the entry number is irrelevant, just make sure you add all the highlited values and the entry numbers are valid, by entry numbers I mean 3, 2 and 2 in this case but they can be different). You could use an online editor if you really like the tool you curently have and don't wanna re-craft a new one. The player file is inside the world file inside playerdata.

[sireshades]

A little late to this discussion, but if you hold the tool (or armor if you have construct's armory installed) and use the command /levelupTool
Then it will set a tool with no leveling to level 1 (0xp) and then the mod will function normally.