Not "single bubble" but "singles bubble"
"Singles" are single people. People who don't have spouses.
A bubble is (in the context of Covid) a group of people who may make social contact with each other, but must socially distance from people in other bubbles. When a case of covid occurs within a bubble, only those in the bubble need to isolate, not the entire community. The formation of "bubbles" is one strategy that governments and organisations have used to allow for some groups of people to meet during lockdows.
So a "singles bubble" is a "bubble" (in the above sense) for single people