Fix

The error "file already closed" occurs because you are attempting to read from or write to a file after calling Close() on it. Ensure all read or write operations happen before the Close() call, and check that you are not accidentally closing the file in a loop or defer statement before you are finished with it.

f, err := os.Open("data.txt")
if err != nil {
    log.Fatal(err)
}
defer f.Close()

// Perform all operations here
data, err := io.ReadAll(f)
if err != nil {
    log.Fatal(err)
}

// File is automatically closed here by defer
_ = data

The "file already closed" error in Go means your code is trying to use a file that has already been shut down. Think of it like trying to read a book after you've already thrown it in the trash. You must finish all your reading or writing tasks before you close the file handle.