I've the Power BI Desktop RS Version as 1.5.7074.36177 (May 2019) and Power BI Report Server (Development version): 2.73.5586.1501 64-bit (September, 2019)
I tried to save the report to PBI RS, it is throwing following error:
We can't save this Power BI Desktop report because either it is newer version or has component parts that are not supported.
I was able to upload it previously and I'm not sure what settings have changed. It started giving the above error.
Upgrade your Power BI Report Server version to the latest available to make sure it’s compatible with users using the latest version of Power BI Desktop available.
You should now be able to access and save reports from/to the server as usual from all versions of Power BI Desktop.