A few different things can cause a trade rejection. We’ll start with the small stuff.
Do you have $$ in your account?
Do you have any pending orders? Pending orders affect your buying power and therefore can cause subsequent/additional orders to not go through.
Is the stock market open? Standard hours are M - F from 9:30AM - 4:00PM EST
Do you have a Good Faith Violation from the previous trading day?
Is your account restricted?