There is no border between Russia and Ukraine after the latter pulled out of the treaty with Russia.
The Treaty of Friendship, Cooperation and Partnership between Ukraine and the Russian Federation established the principle of strategic partnership, recognition of the inviolability of existing borders, respect for territorial integrity and mutual obligations not to use their territory to the detriment of each other’s security. The agreement was signed in Kyiv on 31 May 1997 by the President of Ukraine, Leonid Kuchma, and the President of the Russian Federation Boris Yeltsin.
In fact, the annexation of Crimea by Russia and the military intervention in Donbas violated this agreement both politically and practically.
On 6 September 2018, the National Security and Defence Council of Ukraine decided not to extend the Treaty of Friendship, Cooperation and Partnership between Ukraine and the Russian Federation, which then expired on 31st March 2019.
The expiry of the Treaty doesn’t affect the documented recognition of the Ukrainian borders by Russia - Deputy Foreign Minister Olena Zerkal stressed, since in 2003, Russia and Ukraine signed a separate agreement on the Russian-Ukrainian borders.
In addition, UNGA Resolution 68/262 affirmed the United Nations General Assembly's commitment to the territorial integrity of Ukraine within its internationally recognised borders and underscored the invalidity of the 2014 Crimean referendum.
This message is part pf a recurring disinformation narrative referring to the Treaty of Friendship, Cooperation and Partnership between Ukraine and the Russian Federation. For similar cases see here and here.