Are you trying to add a Full-Width section to your SharePoint communication site on the root of your tenant (URL: https://[tenanturl].sharepoint.com) ?
Are you expecting to see these options for your SharePoint page sections:
And, instead seeing these:
The likely reason is that your SharePoint Online root site has been around long enough to go through transition from Classic Team Site to Modern SharePoint Communication site.
Current known workaround is this:
- Create a brand new SharePoint Communication site. You’ll get this URL: https://[tenanturl].sharepoint.com/sites/NewSite
- Swap current root site with the new site you created in step 1 by using this sequence: How to: Swap your root/ main SharePoint modern site with another
This will essentially archive your old root site (the one that didn’t let you have a Full-Width section) and put the new Communications site in its place. Unfortunately, if you had any data on your old root site–you’ll have to move it to the new Communications site.
To enable Full-With section on the team site, follow these steps: Enable the communication site experience on classic team sites - SharePoint in Microsoft 365 | Microsoft Learn