Hello,
There has been many instances of people upgrading to Moodle 3.9 with the Adaptable theme which is still (as of 16:36BST 2nd July 2020) only for Moodle 3.8 and below: https://moodle.org/plugins/theme_adaptable. Therefore your Moodle WILL break and cause you grief. If you want to upgrade then I suggest that you uninstall the theme and transition to one that is supported, make a full backup beforehand, and note down / export the JSON setting values.
Gareth
Moderator