Every time I open a document with math in it, TeXmacs asks the MiKTeX installation on my Windows 10 laptop to get the cbgreek package, which is already installed. This also happens every time I open the font dialog window.
I do not know why this happens, but I would try to install the package in both the user and the shared installations of MikTeX (both reachable through the MikTeX Console).
Maybe TeXmacs is trying to install the cbgreek in administrator mode without having the rights.
I did it, now the package is installed both on the system and the user level, but it keep asking for it.
I doubt it failed due to privileges, it correctly shows the UAC prompt.
Then I have to apologize, I do not have any more ideas. Let us see if someone else can answer.
I too have the same problem. I wish someone had a solution.
I manually uninstalled and reinstalled the packet. Now it works fine.