There are a couple reasons why you may not be able to place a buy order:
You don’t have enough buying power to place the trade. Cryptocurrencies are non-marginable and can’t count as collateral, so you’ll need to have enough cash in your account to place the order.
You don’t have enough equity to reach your margin minimum. If you have a Gold account and are using Gold Buying Power, you won’t be able to place a cryptocurrency order in your Robinhood Crypto account if doing so would cause you to fall below your margin maintenance or minimum balance in your Gold account. Also, in a Robinhood Instant or Gold account, you cannot place a trade that would cause you to fall below your pattern day trade minimum equity.
Your account is being prevented from buying stocks. If we’ve placed a buying restriction on your account for stock purchases, you’ll also be unable to buy cryptocurrencies until your restriction has been lifted.
Your account was flagged for Pattern Day Trading. If you’re a pattern day trader, and the purchase of a cryptocurrency would cause the value of your stocks and cash to dip below $25,000, we may prevent you from placing the trade. For example, if your account value is $25,500 and you want to place a $600 buy order for Bitcoin, we’ll ask you to deposit more funds. This is because placing the order for $600 for Bitcoin would drop your account down to $24,900.