Your screenshot shows that you are running 1.8 but not 1.8 legacy as required. Can you try again using the legacy download links as provided in the link you supplied in your initial post? This is the section you need to look at:
as explained I downloaded the file and followed the instructions for ROCK :
1. Go to the /Data/ share (instructions) and create a folder called branches
2. Download the following file: roon_1-8 (It should be named roon_1-8 and saved with no file extension)
3. Copy the file into the branches folder previously created
4. Rename the file to roon (instructions on how to rename files: Windows / MacOS)
5. In the Web Interface, click "Reinstall" under Operating System
As I understand your instructions, after the reinstall step, the core should be on the legacy branch but it is not despite the content of the roon file.
Yes, I connected to the Rock WebUI and clicked on the Reinstall button under the Operating System section. I even did twice but nothing changes.
For whatever reason I thought that Roon Server must be shutdown before. I think I was confused by the 1.8>2.0 instuctions just below. Sorry for that.
I tried once again but this time it worked after I unauthorized my account on the core :