FetchApp | GAS library for creating and requesting the type | GCP library

I successfuly saved a string to sharedpreferences. But, I dont understand how to getString and use this a parameter llike below code.

My getstring method;

I have the simplest set of code which does not seem to be working on Flutter. It is a simple for loop, which seems to be giving the exception.

I'm getting an e-mail daily with an XLSX file that I need to then export over to a specific Google Sheet. The code simply searches the GMAIL for a specific query, where it then gets the body (which contains a URL), which it then downloads with the doGet function, that uses the Url.FetchApp.fetch(url).getBlob()

After that, using the variable fileInfo, it defines, the title, mimeType of the file and the "parents" which is the ID of the folder where I need to deposit the converted file.

Then, I insert the file with the Drive.Files.insert(fileInfo, file, {convert: true}) which should convert my Excel file and deposit it in my folder. The export works (a file is exported). If I append a .xlsx extension manually and try to open in Excel,it opens perfectly with all the required information.

But, the issue is that I don't need to open in an Excel file, so I'm trying to export it to a Google Sheet or even parse it as a CSV, but it doesn't work.

When trying to open this exported file by ID, it says that it is missing or don't have permissions, but it is there and I do have permissions. When checking the mime type of the file, it says "application/x-zip".

Is there anyway for me to upload the file data to my Google Sheet with the ID "1HMXgJuuRFaGK11sfR38mKh6rk4ta_Qgtlljk6HBjLkE"?