Can I use an adapter to plug a Nespresso into a NEMA 6-20R outlet?

That NEMA 6-20R is a 240-volt outlet, which is definitely not compatible with a standard North American Nespresso Vertuo. Most of those machines are rated for 120 volts, and if you use a simple physical adapter to force the plug in, you'll be sending double the intended voltage into the machine's electronics. It will likely fry the circuit board or the heating element instantly, and yeah, it could absolutely be a fire hazard.

The reason you have that outlet is probably because a previous owner had a high-powered espresso machine or a heavy-duty countertop oven imported from Europe. While 120V is the standard for most US appliances, high-heat devices often perform better on 240V. But unless your specific Nespresso model is a European 220-240V version, an adapter is a huge mistake. Your best bet is to have an electrician convert that circuit back to 120V. Since it's already a 20-amp circuit, the wiring is likely 12-gauge, which is perfect for a standard kitchen outlet. The electrician would just need to swap the double-pole breaker in your panel for a single-pole breaker, move one wire to the neutral bar, and replace that 6-20R receptacle with a standard **NEMA 5-20R** (the one with the horizontal slot that also takes regular plugs). It is a relatively quick job for a pro since the wires are already pulled through the walls. Don't try to solve this with a cheap travel adapter or a voltage transformer. Transformers that can handle the high wattage of a coffee machine's heater are bulky, expensive, and often unreliable. Converting the outlet properly is the only way to make that spot safe for your new machine.

Listen, if you follow the advice to swap the breaker, make sure you're grabbing a 20A GFCI like a Leviton GFRNT or a Legrand Radiant. Since it’s a kitchen, NEC 210.8 requires that protection. If you open the box and find a red wire, it means you've got a multi-wire branch circuit, wich gets a bit more involved with the handle ties. I’ve seen people try to run those Vertuos on cheap 2000W travel transformers—don't do it. The inductive load on the pump and that thermoblock heater will eat a cheap transformer for breakfast. Pop in a Square D QO120 breaker and do it right.

Listen to these guys. I remember back in the 70s when people woud try to 'cheat' 240v lines for small stuff and end up smelling ozone and burnt plastic five minutes later. If you open that box and find those wires are old-school cloth-covered or wrapped in that stiff, greasy paper, be extra careful not to crack the insulation when you're moving things around. Back then, we'd just tape 'em up, but nowadays you realy want a clean connection. If that 6-20 was for an old window AC unit, check the panel to see if they doubled up any other neutrals while they were at it. Better to do it right once than chase a ghost in the wall later.