A better translation of 미국 is United States because America is not specific to the country. North and South America are also "America" even though an "American" is usually a person who is a citizen of the United States.

"I go to the USA once a year" should be accepted.

It should also accept "I go to the United States once a year". It is infuriating to keep getting answers wrong when you absolutely know what it means and can provide multiple correct translations.

this sentence never implies that it is I who is going to America. This could be interpreted as an imperative stated to someone else. therefore, a translation should be accepted that doesnt specify either: "Go to America every year" or "I go to America every year"