I test to backup some of onedrive and get warning download wait timeout exceeded. What mean of this error and how to fix this?

 

Did you try using Google?  It is exactly what it says a timeout.  If you need more details a little research goes a long way.

What kind of internet connection are you using and where exactly is your M365 backup server located? Could be a timeout that is caused by your internet connection or a firewall misconfiguration.

In case you cannot find a solution I recommend opening a Veeam support ticket.