Hi Mikhail
The 20200615XX release series is Moodle 3.9
https://moodledev.io/general/releases
So the version of Lambda theme you are trying to install will not work on a 3.6/3.7 that you have tested on.
Richard
The 20200615XX release series is Moodle 3.9
https://moodledev.io/general/releases
So the version of Lambda theme you are trying to install will not work on a 3.6/3.7 that you have tested on.
Richard