Labour can potentially provide some capital but the mechanism by which it has to do so in the socialist system would be the wildly impractical an equal amount from each labourer. Anything that causes investment to break one worker, one share for the ownership structure of the business is not allowed. This means if some workers have enough capital to invest they aren't going to do so since it doesn't give them any practical returns. Only if all workers can agree on what to invest and all workers can and will do so will the required ownership structure work with investment from workers.