If you are a mac user and you tried to enable GTM preview mode in safari but didn’t work that is because Apple has Introduce Intelligent Tracking Prevention
In Safari, third-party cookies are blocked by default, that’s why Google Tag Manager Preview Mode does not work.
Steps To Enable GTM Preview mode in the Safari
Go To Safari Preference
To change the setting of safari, you have to open Setting.
Open Privacy Tab
In setting, Click on the privacy tab.
Uncheck Website Tracking
You have uncheck the website tracking and close the setting.
Now you can open the Google Tag Manager and Verify it, Google Tag Manager will be working on your Safari.