Depends on what type of Tactical Nuclear Weapon the Russian uses.
If this is sub-kilotons, most likely NATO will be seriously condemning such attack, likely to intervene directly somehow (like enforcing a no fly zone), unlikely to send troop to Ukraine.
If this is multiple kilo tons of tactical nuclear weapon (below 20kt) Then most likely NATO will directly intervene, possible troop on the ground or peacekeeper, this is most likely ends the conflict for Russia. This is because if Putin uses kiloton Tactical Nuke, anything east of the Dnieper River would have blown toward Poland, and that would trigger Article 5. And depends on Russian response, they may escalate or they may call it a day. Also, if they use 20kt device East of the Dnieper River, more than likely the Radioactive Fall out and radioactive matter will blown back toward Russia.
On the other hand, Russia using Sub Kiloton nuclear weapon is low, because there are a lot of ways to deal more damage in Ukraine than using them, using them is more of a statement rather than actual usage, it won't change the situation on the ground, in fact, even using 20kt weapon won't change the balance, as it can at most take out a single city. You will need 5 or 6 or even 10 to change the situation, if you use ten 20 kiloton nuclear weapon, that's starting a global thermonuclear war.