Which kind of an app is this - eg: private created on an admin, or public/ custom created through the CLI? Have you made any changes to scopes?
This is a private app created on an admin. We didn't make any changes to scopes. Right now, this problem has solved by our technical staff. The reason why we are unable access to the Shopify API is the website was blocked somehow. And as far as I know, this issue was happened on many websites. And this issue does cause a lot of trouble.