You can say basically whatever you want in the United States, with few exceptions.

‘Inciting imminent lawlessness’ aka starting a riot is not protected speech, making threats against the US President is also illegal. There might be other items of speech that are forbidden that I am not remembering.

You can be sued for defamation, but it’s very hard to prove as you must prove the speaker knew what they were saying was false, which is hard to prove.

But lies? You can lie all you want as a private citizen and the government cannot stop you.