That's when they are trying to buy respirators from American companies, and these companies have already agreed to be making them for American hospitals. These companies said "we'll make more for american hospitals next, but we want to sell this first batch to other countries."
Defence Production Act says that, when it is a big emergency, the US government has the ability to force American companies to produce products that are needed to fix the problem. This virus has been declared as that big of an emergency, and the Defence Production Act is now being used. Most companies already volunteered to switch their factories to produce medical equipment, so they did not need the law to make them do it. But this law says "We need you to make as many of these machines as fast as you can, because we need to supply them to the hospitals." So when one of the companies wants to sell a huge number of these machines to another country, they are not allowed to do that because America needs them first. When american hospitals are all supplied enough that they can care for all of their patients, then American companies can start giving the rest to other the countries.
It's an emergency situation, each country needs to take care of themself first.