An administrator in your account has to activate this in your settings. It can be found in settings > CRM > Fields > Order & opportunities. 
This will let users choose their own probability on an opportunity.
Permission needed:
Articles in this section
- Q: Why can't i choose my own probability for a opportunity stage?
- Sales Process BASIC
- Won/lost buttons
- Competitors overview
- Solution-focused selling
- 3 steps to the right timing
- Sales process guide: Reasons to verify your lead's budget
- Defining stakeholders & job titles in your Sales process
- You hold on to the opportunities you lose 3X longer than the ones you win
- Why it's important to always have a next step planned