Currently, any files stored in a Teams Private Channel (i.e. in the SharePoint document library created for that Teams private channel) cannot be touched at all by Excel actions or triggers in Power Automate. When you create an Excel action or trigger, you do not see the private channel's SharePoint in the dropdown list of sites. Entering in the url as a custom value seems to work because you can then select the library name and even navigate the folder structure with the file picker pop-up, but you cannot view or select any Excel files. Even here, if you manually enter the folder and file path, you cannot get the Table list to drop down. Here again, if you manually enter the Table name, the flow will simply fail.