As you can't post anywhere, neither on the app nor on the website, your account is maybe restricted from posting comments.
Try spacing out your comments. Don't include more than 5 mentions and 30 hashtags in a single comment. You can't also post the same comment multiple times, even emojis.
Now, I advise you to avoid posting comments for the next days. After 3-4 days, you can try again - and if that persists, report the problem to Instagram.