With HIV the virus basically highjacks the patients Immune cells called T-Cells via the CCR5 receptor and changes the functionality of them into a virus production cell, basically killing The T-Cells, hence the immune system declines overtime without the appropriate drugs or possible drug (Leronlimab).

I’m pretty sure Covid-19 doesn’t enter T-Cells and hence acts in a completely different manner and a vaccine is possible.

I’m not saying a vaccine for HIV is impossible but it would have to stop the virus from entering billions of CCR5 receptors and not miss one of them to be effective. This is why some people with the CCR5-Delta32 T-Cells mutation are immune to HIV.