Google Optimize: Unable to preview experience. Please make sure to enable cookies and restart the browser

This is how it worked for me:

In Chrome go to Clear Browsing Data (chrome://settings/clearBrowserData).

Make sure you select checkboxes Cookies and Cached images and files.

Submit the form by hitting the button "Clear data".

Go to preview experience.

I think I've found a fix for this. Go to the top right corner of chrome (the three dots) Settings > Privacy & Security > Cookies & other data > and add the url in the bottom section called "sites that can always use cookies"

Example

For me it started working when I gave the Google Optimize plugin access to all pages (not just the page running the experience).

This happens to me quite often too. The suggestion Google gives is wrong. Clearing cookies and restarting the browser does not fix it. Sometimes rebooting does, but that's a huge disruption.

What I found to work pretty reliably is to end the preview (dropdown), then click the EDIT button to go to the editor. Come back to the setup page, then try Debug again. Debugger should work again.

If you are using chrome then:

• Click the context menu in the browser toolbar to the right of the address bar.
• Choose Settings.
• Select/Search the "Security and Privacy" section.
• Choose "Cookies and Other Site Data" Ensure that "Allow all cookies" is selected.