Hi Michael,
By default, the S5 Fitness Center template does not override the article layout:
root/templates/fitness_center/html/com_content/
It seems that in Joomla 3, you created a custom override in this folder. After migrating your site to Joomla 5, this override must be updated to ensure compatibility with Joomla 5 and the latest PHP version.
Please review all PHP files in the following folder on your site:
root/templates/fitness_center/html/com_content/
Update them as needed. Alternatively, you can temporarily rename this folder to confirm that the error is caused by the override. Renaming it will disable the override, allowing you to see if the issue persists.
Best regards