This is a recurrent pro-Kremlin disinformation narrative about war in Ukraine.
NATO countries are not sending mercenaries to Ukraine. Relations between NATO and Ukraine date back to the early 1990s and have since developed into one of the most substantial of NATO’s partnerships. Since 2014 the cooperation has been intensified in critical areas. NATO has reinforced its support for capability development and capacity-building in Ukraine. The Allies continue to condemn Russia’s illegal annexation of Crimea, and its destabilising and aggressive activities in eastern Ukraine and the Black Sea region. NATO has increased its presence in the Black Sea and stepped up maritime cooperation with Ukraine and Georgia.
The Lithuanian-Polish-Ukrainian Brigade really exists. An agreement on its creation was signed in 2009, and the unit was finally formed in 2014. The task of the brigade is to participate in peacekeeping operations and to strengthen cooperation in the region.
It is actually the unrecognised separatist quasi-states in Donbas who actively involve mercenaries from Russia and other countries, see for example, the well-known case about Chechen fighters. In addition, Russian active-duty soldiers serve there, too. Some of them have been captured and admitted to their missions in Ukraine. Moscow calls its mercenaries "volunteers" because some of them go to Donbas on holiday. The Russian Defence Ministry says that is not responsible for their actions off-duty. Russian President Vladimir Putin said at a news conference in 2014 that they are volunteers and not mercenaries "because they do not get money".
Read more disinformation cases about Ukraine allegedly hiring foreign mercenaries for action in Donbas.