why can't I create board in some workspace

[Wax Anan]

I can't create board in some workspace only. The Create bottom is not green to click 

[Gaurav Kataria]

You are likely a guest in that workspace. You can request to be added as a member, which the admin will need to approve. Details here https://support.atlassian.com/trello/docs/request-workspace-membership-as-a-guest/