I tried to install the latest firware. It will not install. I also do not see any light on top glowing. So I have no idea what to do now.
which firmware level are you trying to install ?
I had troubles to install 2.6.6 a few days ago.
Support told me
a) to restart the smarphone (I’m using Android 9) and
b) factory reset Nuki SL 2.0 (i.e. removing batteries and put them back again with push and hold the button until the LED ring glows.
I also had problems to update to the last two firmware versions. It took me around 10 minutes each until I succeded after several connection errors, retries and reboots of the smartlock.
I have the same problem, but I think I figured it out.
When you are prompted to press the button to enter update mode, the full circle lights up instead of the 1/4 circle described.
At this point you have to take your finger briefly off the button and press and hold it again. Then the light ring changes to the 1/4 circle and the installation process begins.
The description in the app is simply wrong.
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This should make no difference at all.
Thank you so much. Your procedure made me finally upgrade the lock’s firmware. I have been struggling so long.
@MatthiasK Please try this procedure once again. Once your way (no upgrade), then the alternative way. The difference is huge 
Agree! It worked for me as well.