Please first double-check that a blocker mission is not active in your missions window. While a blocker mission is active, you won’t be able to accept new missions of that type until it expires. The blocker will be incurred whenever you complete or delete a daily mission.
Sometimes, a known issue can prevent you from accepting new daily missions with no blocker present. This issue most often occurs when a player zones while accepting or completing a daily mission. If you’ve been affected by this issue, please submit an in-game petition using the /petition command in your chat window. A Game Master will be happy to help you proceed.