I recently had to setup a new Office 365 tenant.
One of the first things I noticed was that SharePoint Designer was disabled, and could not find an option anywhere to enabled it.
Another problem caused by this is trying to add code snippets to pages, ending up with the message:
"This HTML cannot be inserted because you don't have access to add scriptable Web Parts in this site. Please contact your administrator if you think you should have rights to do so."
After searching for a while I found a solution using SPO.
1. Download and install SharePoint Online Management Shell
2. Load up the newly installed SharePoint Online Management Shell console
3. Run the following code snippet (after setting up your tenant details)