Hi @Anonymous
With regards to the first question, it sounds as though PowerApps cannot find the Excel file that's associated with your app. This can happen, for example, if there's no personal or business OneDrive account associated with your login.
To resolve this issue, I would suggest that you recreate the app and choose the option to manually select the location where you want to save your Excel file. You could choose to use Google Drive or Dropbox rather than OneDrive, which may work out to be more reliable in your situation.
Regarding your second question, if you carry out the tasks in the actual designer, the Next button will appear which will then allow you to progress to the next screen. For example, I've highlighted the section in the properties pane which will answer the first question.
