Hi,

The "AssertionError: Element odoo has extra content: template" during Appointment module installation indicates an XML structure error. The <template> element is incorrectly placed directly inside the <odoo> element in one of the module's view files.

To resolve this, identify the problematic XML file (likely in the views directory), and enclose the <template> element within a <data> element. This corrects the XML structure to comply with Odoo's schema. Save the changes and retry the module installation.

Hope it helps

Hello,
If you have a custom module then check inside the xml view. or try upgrade website app. and still not resolved verify all required dependency modules for Appointment are properly installed and upgraded.
Hope it helps.