Hi there,
Sela has been retired on WordPress.com, yes, but that’s not in any way related to the error you’re seeing. I’m able to install and use Sela without any problems on my own site, so whatever is going wrong here is specific to your site.
When reinstalling, did you completely delete the Sela folder from your site via FTP? Or did you just delete it in WP-Admin? Please delete the folder completely before you try reinstalling.
If reinstalling the theme did not help, the error might be caused by a conflict with something else on your site. Please see if disabling all your plugins make any difference. If it does, reactivate your plugins one at a time until the error returns to determine which plugin is the problem.
Any ideas about what theme to use instead of Sela
Sela has been replaced with Stow. Stow has the same design, but has been rebuilt completely for the block editor. You can find a download link for the self-hosted version of Stow on that theme’s page on WordPress.com:
https://wordpress.com/theme/stow
Any comments about why they do not want to support Sela any longer? I think it is a great theme
Themes are regularly retired when they become too old to maintain. And in this case we’re replacing the theme with one following the same design, but optimized for the block editor, and it’s too much overhead to continue maintaining the original version of this theme along with the updated version.