Flagging bans are described in this post on Meta Stack Exchange.
If at least 25% of your flags from the past 7 days were declined, you're blocked from flagging anything. Depending on when and how those declined flags were cast, this block could last for as little as a day, and won't ever be longer than a week.
So sometime between now and 7 days from now, you'll be able to flag posts again. This reflects the fact that a flag ban isn't really a big deal.
For now, I suggest reviewing your flag history and the guidelines for flagging to learn how to target your flags better. If you have any questions on how to use flags, you can ask here on meta or in the chat room.