You might be trust level 0.
From the syntax and features page:
Trust Levels
Trust levels are a participation system on the forums. The more you post, like, reply, read, and visit the forums, the more you work your way to a higher trust level. Increased Trust levels grant additional privileges. Everyone who is new starts off at Trust Level 0 but will quickly climb by reading and visiting the forums within their first few days.
TRUST LEVEL 0
Trust level 0 users cannot:
- Flag posts
- Post more than 3 topics
- Post more than 10 replies
To see how to increase your trust level: