The public debt is just the accumulate of past deficits. If the Fed is owning the debt you are paying interest to yourself. The only risk is inflation and this

The public debt is just the accumulate of past deficits. If the Fed is owning the debt you are paying interest to yourself. The only risk is inflation and this is not something to be worried at the moment. MMT is only a description of how the systems work. But if by "test it", you mean that the Central Bank is backing the public debt, Japan have been doing for, at least, twenty years.