One problem with this question is that UBI can be implemented in different ways and the way that it is implemented is very important.

I think that the way most people think about UBI is that you would get enough money to not have to work. I don’t think that this is compatible with capitalism, because the main reason why people work is because they are pressured into it for economic reasons so removing that without providing people with some other reason to work will just cause the economy to collapse.

Even if people work for some other reason than money, you will still have the problem that UBI undermines itself. As less people work for money, the money you get from the UBI program will also mean less. Not only do you need a different way to encourage people to work, but you also need a new way to distribute the products of that work if you want to ensure that everyone has access to basics like food and housing.

For these reasons I don’t think that a UBI that offers people the option of not working is compatible with capitalism. Capitalism is the system that we use to distribute work and resources and if we implement UBI we will have to invent new systems to do those things instead.

It is still possible to have a smaller UBI under capitalism if your goal is to for example prevent money from getting to concentrated among the rich and instead stimulate the economy, or something.