This is correct. In addition, fiat banks have been known to reverse transfers to Coinbase once they realize it is a cryptocurrency exchange. OP, check with your bank. Did a card transaction decline? Did a payment get reversed? Either way, if Coinbase are not allowing you to top the account back up to come out of the red, then their support are the only people who can help right now.

Coinbase are an awful exchange, and their support is no different. In the past, other users have had to wait weeks or months to get simple things resolved. The best you can do is keep your issue on their radar. Keep sending tickets. Try PMing some of their staff who are active on reddit: https://www.reddit.com/user/coinbasesupport and https://www.reddit.com/user/justin_coinbase. Make some Twitter posts about it. Usually they are only actually motivated to help customers recover locked funds or accounts if it looks like there will be community backlash - they don't actually care about helping you get your money back.