Hi @Victoria ,
Which PowerApps licenses does this user own?
Is the user existed within same Org as you?
Based on the issue that you mentioned, I have made a test on my side, and don't have the issue that you mentioned. Please make sure the user has enough permission to access your SP list data source, and could access your PowerApps app.
Also please check if the user has appropriate license to run a PowerApps app. If a user wants to run a PowerApps app, he must own Office 365 include PowerApps license at least.
Please check the following article for more details:
https://docs.microsoft.com/en-us/power-platform/admin/pricing-billing-skus#powerapps-for-office-365
Please also consider ask the user to clear cache of his browser, then access your app again, check if the issue is solved. Also please make sure the version of the PowerApps Mobile device in his device is the latest version, if not, ask him to upgrade it to the latest version.
If the issue still exists, please consider remove the permission to your SP Site and your canvas app for the user, then re-assign corresponding permissions to him, then try it again, check if the issue has been fixed.
Note: Please make sure the user is in same Org as you. In other words, you could not share an app with users outside your Org (external users).
Best regards,
