Issue Creating Pages in Subsites on SharePoint Online - "Invalid Field Name" Error

Hello,

SharePoint 2013 to SharePoint Online is quite a big leap. My recommendation would be to try to replicate what has been built as a customized solution, but in a modern way. In best case, try to use out of the box functionality to achieve your goals. 

If you rely on classic sites for your current migration, there is a big chance that you would need to migrate again. I do believe that the classic sites will not have so long lifespan and that there is a risk that they will become unsupported by Microsoft in the future. 

For new tenants (last two years), the root site is by default SITEPAGEPUBLISHING#0 (communication site) or on some rare occasions team sites. In my experience, I do not believe it is possible to convert it back to classic site. 

Yours sincerely,
Aref Halmstrand