Is Roku OS available for the model: 3900X - Roku Express? It was not listed as a supported model for Roku OS, although similar-sounding models like the 3700X were listed. I believe the 3900X was introduced in 2017, but it was at the low end of the price range for Roku models.
Be patient. Roku rolls out the updates in waves to various users and models and monitors for problems. If you look back a bit in history, you can see all the issues 9.2 had with HDMI handshaking when it rolled out.