It's not true. I'll give just one example, but there are others.

In April 1980, President Carter ordered an armed rescue of the American hostages seized from the American Embassy in Iran during the Iranian Revolution. Codenamed Operation Eagle Claw, this was intended to be an intricate airborne assault on Tehran that would quickly grab the hostages and fly them out of the city. It ended in a debacle.

While Carter insisted that the raid attempt to use non-lethal weapons such as tear gas against resistance, this was just a fig leaf. As the article above indicates, very few nonlethal munitions were carried on the raid, and until it was aborted in disaster, it was a significant military operation that likely would have resulted in large numbers of casualties.

Carter supported Eagle Claw, and had been preparing for an armed rescue since shortly after the embassy takeover. It was only in early April that he decided to execute the military option.

Carter may never have "fired a bullet" through US forces during his presidency, though his actions certainly allowed other nations (often dictatorial despotic regimes) to "fire bullets" which they obtained in large quantities from the Carter regime. He seemed to prefer non-intervention, while allowing these regimes to slaughter people using US-supplied weapons and ammunition. I'll refer you to William Blum's book "Killing Hope: U.S. Military and CIA Interventions Since World War II" for some details. In Zaire, it is true that there were no US-provided troops involved, but US-trained foreign forces (several thousand Morroccans) were involved, and Carter did not object when our European allies sent in large quantities of lethal supplies. Also in 1977, there was Guatemala, which received weapons and military equipment from Israel, at least part of which was secretly funded by the USA. This weaponry allowed a genocidal campaign against indigenous and anti-government forces. Then there was the East Timor genocide, in which the Indonesian government was fully armed and equipped by the US. Afghanistan is another war in which we supported one side against the other, as was El Salvador in 1980. So, technically perhaps he did not "fire a bullet" during his presidency, but his hands are certainly dirty.