I ran into this one today and it's the in the category of things that I'm likely to run into again one day so it's worth recording it here.
I was trying to configure an unattended execution account for SQL Server Reporting Services 2016 and was getting the above error.
The problem was that even though the machine is just in a workgroup, SQL Server Reporting Services Configuration Manager needs a domain or machine specified for the user name.
You can't just set a name like SSRSUnattendedExecution, it does have to be MYMACHINENAME\SSRSUnattendedExecution. That's different to the other SQL Server services that happily let you configure it that way when setting them up.
The message is misleading as it suggests that you can't do it right now. You can't do it ever like that.