This problem is on rapidapi side, not api. Can you, please, check that request config is similar to example:

curl --request POST \
  --url https://paraphrasing-and-rewriter-api.p.rapidapi.com/rewrite-medium \
  --header 'X-RapidAPI-Host: paraphrasing-and-rewriter-api.p.rapidapi.com' \
  --header 'X-RapidAPI-Key: ***********************' \
  --data '{"from":"en","text":"New York, often called New York City or NYC, is the most populous city in the United States. With a 2020 population of 8,804,190 distributed over 300.46 square miles (778.2 km2), New York City is the most densely populated major city in the United States and more than twice as populous as Los Angeles, the nation'\''s second-largest city."}'
	```