Hello Bhaveshp Patel, welcome to Microsoft Q&A forum.
The message means your system is missing important Windows files and needs proper installation or recovery media to fix the issue. Windows 8.1 reached end of support on January 10, 2023, so Microsoft no longer provides the download link directly. If you need the media, you can still find Windows 8.1 or Windows 8.1 Pro installation files on reputable third-party sites. Just make sure to use trusted sources to avoid modified or unsafe downloads. Since the operating system is no longer supported, it’s also a good idea to consider upgrading to a newer version for security and updates.