To be able to add Initiatives to an OKR, you need to be a member of the Group the OKR belongs to, be the lead of the OKR, or be made a Contributor for that OKR.
Find an overview of roles and permissions here.
You can find the Lead and Contributors of an Objective on the right-hand side:
Need a hand? Contact our friendly Support team via the in-app chat at the bottom-right of your screen, or email email@example.com.