Importing a file into the App leads to a file error message

When you try to import a file from a virtual location such as a remote drive or a cloud storage (iCloud, Dropbox, Google Drive, etc.) the App might not be able to obtain the necessary permission to access that file because of all security restrictions applied by the operating system (iOS or Android), so the best practice is to copy the file into the App's own document directory and import the file from there.

Once you're sure that you don't need the file anymore, you can delete it from there to avoid filling your storage device with duplicate files. Only if the App needs to access the file automatically every time it is launched, the file must be there.