The os-prober isn't executed. There is a Way that I know what You could try. Open the Terminal and type sudo nano /etc/default/grub
Then You will get something like this:
In this white Block You set at the End the following Line:
GRUB_DISABLE_OS_PROBER=false
Then it should look like this:
When You have change that, press ctrl+o to save the Change, Enter to confirm and ctrl+x to exit. Then type sudo update-grub