Here's one of the answers to the question:

"Between 2015 and 2019, Moscow signed 19 military collaboration agreements with African governments. The collaboration has focused in large part on Russian weapons sales."

https://www.brookings.edu/blog/order-from-chaos/2022/02/08/russias-wagner-group-in-africa-influence-commercial-concessions-rights-violations-and-counterinsurgency-failure/

Cuba wholeheartedly supports Russia for the same reason, and because of their shared history.