The reason Spain didn’t come into the war on the side of the Axis is because it was so weak that it would have been a drag on Nazi resources. The same is true of Belarus. They have a very small poorly equipped army. The Russians would have to carry them, and the Russians can’t even carry themselves. It would add another front on a front line that is already practically indefensible. Belarus is keeping a large contingent of Ukrainian troops tied down.
As for Belarus, didn't they permit Russian troops to invade Ukraine from the north in an unsuccessful attempt to take Kiev?