Under the Merchant Center login tab (in the Manage Linked Accounts page), you will see a list of the feeds to which the Merchant Center login email has access.

On the page to manage Google Merchant Accounts (after picking a Google Ads account), you can find all the merchant feed ids available.

We show "Access not found" when the email doesn't have access to a merchant ID and or we've lost access to it.

To fix this, you can use the link provided in the Action column.