You are view the lightning page from preview? if yes, the toast its not working beacuse the event is handled by one.app.

This event is handled by the one.app container. It’s supported in Lightning Experience, Salesforce1, and Lightning communities.

You can create your own toast component if you want to preview the page, or simply create a tab for the lightning page.

If you need more info check this link of the doc

force:showToast

Greetings.

You are view the lightning page from preview? if yes, the toast is not working because the event is handled by one.app.

Quoting from documentation:

This event is handled by the one.app container. It’s supported in Lightning Experience, Salesforce1, and Lightning communities.

You can create your own toast component if you want to preview the page, or simply create a tab for the lightning page.

Check documentation for more details: force:showToast