Make sure that your all follows all requirements of being a channel : https://shopify.dev/concepts/channels
Sounds like either your app is not embedded, or you are requesting the wrong type of scope. Are you creating a sales channel for a marketplace or other channel, or looking to interact with the Storefront API for your own website?
We are trying to fetch storeaccesstoken and ours is not a sales channel, but want to interact with StoreFront API for own external ecommerce website. Any suggestions on how to get this so that we can create shopify checkout weburl?