No full width column option in subsite?

Lee-Martin What is the outcome goal you were trying to get to when you created the subsite in the first place? Modern site architectures are "flat" - so communication sites are not designed to have subsites. In fact, you cannot create a communication site that is a subsite so if you did create a subsite, it is a team site. As a best practice, I would not recommend creating subsites - even if you technically can. (I almost always disable subsite creation at the tenant level.) Instead, I would ask why you created the subsite? If you are looking to make a relationship between two sites, consider making one a hub and associating the other site to it. That way, both sites are communication sites - with the benefit of the full width section you want - and you can share navigation and search scope and theme without having to use subsites. But, you get the benefit of governance and permissions independence. If all you want to do is surface content from Site A on Site B, you may be able to use the News or Highlighted Content web parts on Site B and select Site A as the source. See: Planning your SharePoint hub sites - SharePoint in Microsoft 365 | Microsoft Learn. For fun, you can also take a look at Goodnight, Subsites a holiday story for IT (microsoft.com).

Lee-Martin 

How did you create the subsite? Through the UI?
Then you most likely created a team site, not a communication site.

Full-Width sections are not available on team sites.

There does not seem to be an option in the UI to create a communication site as a subsite.

So you need to use PNP PowerShell to connect to the parent site and execute these commands:

$Locale = 1031 #1033 English /1031 German
$SiteUrl= "MySubsite"
$Template = "SITEPAGEPUBLISHING#0" #Communication Site

New-PnPWeb -Title $SiteUrl -Url $SiteURL  -Locale $Locale -Template $Template 

Best Regards,

Sven