While this is partially true:
1. Sovereign nations absolutely can and do default. See, Russia in the 1990s and the LTCM fiasco.
2. Even though they do not have to default and can always print money to pay their debts, doing so causes inflation. How much inflation it causes is proportional to how much money is printed.
Now, the dynamics of inflation are pretty complicated, so in certain circumstances you can get away with it for a while. But it is not the case that the government can print money indefinitely with zero consequences.