chabig probably has the best suggestion "try again later". Be sure to restart your Mac before you next try the download, as that seems to make it all work a little more smoothly (might help, won't hurt!
The problem for you - is that the installer doesn't completely download, so you are probably a long download away from deciding that the installer won't install. It's a large download (near 6GB), which will likely take at least 30 minutes, before you can even begin the install.
There are a lot of potential fixes for the error that you see.
Many are listed on
this thread ...
When the download completes, the installer will automatically open.
I suggest that you close it, rather than running the install at that time.
Then, use one of the methods to create a bootable USB installer. I like
DiskMakerX, which makes the task easy, without needing to go into the Terminal (unless you LIKE working in the Terminal, and that's OK, too!
)