You addressed the folder correctly, but then you inserted a space after “Download”
spaces are interpreted as a delimiter so the command thinks “i have do make the file Download and the file ledger-… executable”.
You have to add a / directly behind the folder.
The command you need to run would be chmod +x Downloads/*.AppImage. This will make every file that ends on .AppImage in your Download folder executable.
Just in case you ever have to type a path with a space in the name in the command line: Escape it with \. This tells the shell (the thing that interprets what you write into the terminal) “the space the follows this \ is not a real space, but part of a name”. So if you want to make all AppImages in the folder “My Applications” executable it would be chmod +x My\ Applications/*.AppImage