Brokerage cash may be margin used, and if you have buying power through margin that would be why you can't withdraw it.

Robinhood can't withdraw buying power. By this rule, these uninvested funds must stay in your account for at least 5 trading. Assuming the money is free to be moved, you’re eligible for five. You have to wait a couple of days if you sell stock in order to withdraw it.

Securities settlement time from the sec. You may not be able to withdraw money while your account is restricted. After logging in robinhood.com, click the “account” menu in the top right to bring up the menu dropdown.

Robinhood also offers a margin account through robinhood gold which lets users borrow. When making a purchase, the broker requires you to have a little more money over the displayed price of the stock you want to buy. I realized why i couldn't.

Idk y but i remember reading about it on robinhood website. If you can’t withdraw money from robinhood, check to make sure the funds aren’t still in the settlement period. So if you added $100 and bought stock with $50, you would have $50 buying power.