'Add Subsite' Option Unavailable--Permissions Error?

Hello, newbie here, 

I'm using Sharepoint Online. I need to add Subsites to my Sharepoint Site to organize it/set inherited permissions, etc. I have full control, including permission to create subsites: 

However, the option is not available from the Site Contents page:

Several colleagues also have full control and likewise cannot create one. I created a custom permissions group and specifically granted myself this permission again, but to no avail. 

Anybody seen this before? 

Thanks in advance!

Alex