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This says that your Kickstart file is broken. Typically, this happens if you upload by FTP in Auto or ASCII mode which will corrupt the file. This is especially true since you are using a Windows computer.

Technical info on what's going on: Windows uses CRLF (two bytes) to terminate a line string, whereas Linux and macOS use LF (one byte). Using Auto mode, the default transfer mode for PHP files in most FTP applications when you are on Windows is telling the FTP client to automatically convert CRLF to LF on files uploaded from your computer to the server, and convert LF on files downloaded from the server to your computer back to CRLF. This was necessary in old versions of Windows (we're talking 20+ years ago…) which couldn't open files with LF line endings. Windows 10 and 11 never had that problem. Either way, your FTP client tries to convert CRLF bytes (0d0a in hex) to a single LF byte (0a in hex) present anywhere in the file being uploaded. Since Kickstart is actually a binary PHAR archive file with only a small executable "stub" in front to let it "run", this conversion breaks the archive. It's also the same reason we tell you never to use Auto or ASCII mode when transferring backup archives to and from your site. The conversion modifies the file, breaks it, and invalidates its signature which consists of a simple SHA-256 checksum of the archive contents. Therefore, PHP correctly complains that the file's signature is broken and refuses to proceed any further.

Download Kickstart again. Make sure you are downloading version 9.0.1.

Extract the ZIP archive on your computer.

Rename the kickstart_pro.php file to something different that does not include the terms akeeba, kickstart, or ks. Let's say uh4Q8yhfsYKzKZ4m.php (randomly generated filenames are better for security; please read the documentation for more information).

Upload the file to your site. If you are using FTP / FTPS you must explicitly use the Binary transfer mode in your FTP client. Better yet, use SFTP or your host's file manager which do that automatically (no explicit configuration required).

Let me know how that works for you.

Ah, I forgot the simplest alternative. Upload Kickstart's ZIP archive and extract it on the server using your hosting control panel's file manager. ZIP files transferred in Auto mode are actually uploaded as if you had explicitly selected Binary mode, therefore they don't break. That's why this method will work.

Another alternative is to rename kickstart_pro.php to kickstart.bin and upload it, then rename it back to a filename with a .php extension. This also works because it tricks your FTP client into using implicit Binary mode even though it's set up to use Auto mode.