Well, a lot of things. Despite the government being able to spend as much as it decides, it cannot create its ideal workforces, nor can it spawn materials like steel, oil, or renewable energy from thin air. The real constraints on the economy related to labor and resources, not money. As shown, if the private market is doing well enough that there are not enough workers to build the powerplant, then that powerplant cannot be built. Similarly, just because there are 7 million people out of work, it does not mean we can suddenly build 100,000 fighter jets without any titanium.
According to MMT, the government should not allocate money; rather, it should be allocating resources. The government should not ask “How will we pay for it?” but “What are we capable of paying for?”, and, perhaps most important, “What must we pay for, no matter the fiscal cost?”