This issue is often caused by an XML file having unexpected characters before the actual content. The error message "Content is not allowed in prolog" typically means there's an invalid character (like a BOM or non-printable character) before the XML declaration (<?xml ...>).

To fix this:

Open the package.xml file in a text editor. Check if there are any unexpected characters or spaces before the <?xml ...> declaration. If there are, remove them. Save the file and try running the code again. If you're not able to locate the issue or the file seems fine, you might want to try regenerating or re-downloading the Flutter project to ensure everything is set up correctly.