BACKUP DATABASE WITH COMPRESSION is not supported on Web Edition (64-bit)

The server has a default compression setting that is used if you don't specify an option in your BACKUP statement. (Why it might allow you to set compression-by-default if your edition doesn't support it I have no idea; unfortunately I don't have a Web edition to test that theory).

Try adding WITH NO_COMPRESSION to your statement.

Solved changing the file name.

The default was NO_COMPRESSION but the .bak file already existed and was a compressed one (created on another local machine with SQL standard, and then copied on this server). In this case the default is always with compression. I discovered because putting no_compression leads me to another error saying the media is compressed.

Changing file name or deleting .bak file solved the issue.