Thieves' tools proficiency can't be replaced with another tool proficiency.

I tried making a halfling rogue character with the criminal background. When I tried to replace the theives' tools proficiency with something else, it wouldn't let me. I tried switching to the urchin background, but I still couldn't replace thieves' tools with another tool.

This is a known and something we are looking into. It's best to handle it manually for now.