AWS OpenSearch failed to fetch

I want to use AWS OpenSearch serveless service. I created a new collection. After the collection is created, when I go to the collection main page, there is a red box with message "Failed to fetch". This box is located between the 'General information' and the 'Endpoint' sections. I get the same error, when I try to create a new index in this collection. Once I submit the creation, the same box pop-ups below the form.

I am not using any VPN and my internet connection is stable. I am logged in as a root account in the console.

Why is it happening? Why can't OpenSearch work out of the box?