Unfortunately, Nicepage does not correctly implement canonical tags in Joomla. Here's the workaround I've found which works well:

Optimized Process for Configuring Canonical and Open Graph Tags in Joomla with Nicepage and TAGZ

For effective SEO and social media representation on a Joomla website using Nicepage, follow these steps:

Configure Joomla Global Configuration:

Navigate to System → Global Configuration in Joomla.
Enable "Search Engine Friendly URLs" and "Use URL Rewriting."

Configure System - SEF Plugin:
Access Extensions → Plugins and edit “System - SEF.”
Add your site’s domain in the “Site Domain” field.

Update Canonical Tag Plugin:

Install or verify that plg_system_canonical_3.1.0 is enabled to correct Joomla's canonical tags.

Disable Nicepage Canonical Tags:

In Nicepage settings, turn off "Include Open Graph meta tags." Despite its name, this setting controls Nicepage’s canonical tags.

Install and Configure TAGZ Plugin:

Install TAGZ from roosterz.nl.

Configure it for optimal OG tag generation.

After configuration, inspect page headers using browser tools or SEO tools to check canonical and OG tags.
Confirm proper tag display across various pages.

Unfortunately, Nicepage does not correctly implement canonical tags in Joomla. Here's the workaround I've found which works well: Optimized Process for Configuring Canonical and Open Graph Tags in Joomla with Nicepage and TAGZ For effective SEO and social media representation on a Joomla website using Nicepage, follow these steps: Configure Joomla Global Configuration: Navigate to System → Global Configuration in Joomla. Enable "Search Engine Friendly URLs" and "Use URL Rewriting." Configure System - SEF Plugin: Access Extensions → Plugins and edit “System - SEF.” Add your site’s domain in the “Site Domain” field. Update Canonical Tag Plugin: Install or verify that plg_system_canonical_3.1.0 is enabled to correct Joomla's canonical tags. Disable Nicepage Canonical Tags: In Nicepage settings, turn off "Include Open Graph meta tags." Despite its name, this setting controls Nicepage’s canonical tags. Install and Configure TAGZ Plugin: Install TAGZ from roosterz.nl. Configure it for optimal OG tag generation. After configuration, inspect page headers using browser tools or SEO tools to check canonical and OG tags. Confirm proper tag display across various pages.