You can make a request to buy investments as frequently as you like. However, you can only make one sale request at a time.
If a given action (such as a request to sell investments) will result in being flagged for pattern day trading, you will not be able to take that action until sufficient time has passed to avoid having your account locked.
If you are prevented from selling out of your whole portfolio due to pattern day trading restrictions, you may still be able to sell out of a single position. For more information on selling positions, check out this article: How do I sell stocks that I hold in my Stock Investing Account?