I would say, you need to be a minimum member rank to be able to have a link in your signature. Check this topic and find all rules and limitations about member ranks:
Forum ranks/positions/badges (What do those shiny coins under my name mean?)and what you are particularly interested in:
There are restrictions on signatures, based on your activity/rank.
- Newbie: No styling (including links) allowed. Max 50 characters.
- Jr. Member: Max 150 characters.
- Member: Links allowed. Unlimited length.
- Full: Color allowed.
- Sr. Member: Size allowed
- Hero: Background color allowed