We're using Jira and Confluence, both the cloud versions and also bitbucket for development of some projects.
I'm creating a "Software project space" within Confluence to link to a JIRA project that I've just created successfully. I've picked the newly created project name from the list and all the fields have been populated including the Space key, which it's defaulted to the same key as the project. All required fields are filled out, however, the Create button is still "greyed" out, and so I can't create the space.
Creating a blank space doesn't have this issue, but I haven't followed through as I don't want to burn the space key.
Since you have the create space permission for blank spaces it likely isn't a permission problem.
The only thing I can think of is your space key being unique. Is there a similarly named space in your Confluence instance that is blocking you from creating?
What browser are you using?
I see a related issue here for chrome:
Try doing it with another browser
Would be possible to verify if your license has reached the maximum number of licensed users? Check this on Confluence administration -> license details and verify the number of current licensed users.
If license is not a problem for your instance then you may try the resolution in the KB article create-space-button-inactive-in-add-space-dialog.
I just tried to create a whole new fictitious project and space, which worked successfully. Maybe someone had previously created a space with that key, then subsequently deleted it, and there are some artefacts remaining somewhere. I did see some older questions talking about it being an issue but that was in the server version too not the cloud.
Ah, yes, found it. Under https://company.atlassian.net/wiki/admin/permissions/viewdefaultspacepermissions.action I can see the key I'm wanting to use. I might be able to recover and reuse it from there.
I had the same problem while piloting Confluence Cloud (we were preparing the migration from server to cloud), and the reason was that the space name and key I intended to create was reserved for the content migration from server to cloud. Using a different space name and key fixed the issue for me.