Hi @zeynepbzdg,
Please visit WP Admin > Dokan > Settings > Page Settings and make sure the dashboard page you created is assigned correctly there. Here’s the vendor dashboard page shortcode: [dokan-dashboard].
Then go to WP Admin > Settings > Permalinks and simply click Save Changes once to flush the permalink structure.
After that, try accessing the vendor dashboard again. This should fix the issue.
Hope this helps!