What's stopping ukraine from using tunnel boring machines in order to go pass russian lines ?

In all seriousness, digging tunnels is a military tactic that gets some use sometimes. The Russians have done it in the southern part of Avdiivka. I haven't heard of a case where militaries used an actual tunnel boring machine, though.

Tunnel boring machines are not just extremely expensive, they're also extremely bulky and highly visible when on top of the ground. If you ship one to where you want to start your tunnel, there is a high risk that enemy intel will spot it, ruining the surprise.

The actual digging is extremely slow, meaning there is no way of knowing whether the tunnel will even still have some tactical use when you're done.

Once you emerge on the other side, it won't be long until the enemy notices your exit and does something about it, like bombarding it, stationing troops there or sending something of their own back through the tunnel. That means even if you succeed, you only have a very short window to do something with your tactical advantage, and said something will very likely be a suicide mission.

Tunneling through the loose mud of Ukraine would not be easy. You can do it if you send a work crew after the machine to immediately build strong walls behind it, but that would be even slower and more expensive.

All in all, it would just be more useful to use either much smaller and shorter tunnels that don't use tunnel boring machines or utilize some form of artillery or drones to achieve the desired effect behind enemy lines.