Hi @Anonymous
PowerApps Canvas app inherits Data Access Permission inherits Data Access Permission from the data source itself. If the end users could add a new record in your SP List directly then they would be able to submit new record via a canvas app to your SP List.
In default, if you assign a "Contributor" permission of your SP List to these end users, they would be able to add new items in your SP List directly. Please ask users to clear cache of their browser, then access your SP List again, add a new item, check if the issue is solved.
If the issue still exists, please consider assign a "Edit" permission of your SP List to these end users, then try it again, check if the issue is fixed.
If this post helps, then please consider Accept it as the solution