Please update the addon for Shadowlands. It simply does not load. I have updated to the latest version but doesn't work.

2x Details\Libs\DF\panel.lua:663: attempt to call method 'SetBackdrop' (a nil value)
[string "@details\Libs\DF\panel.lua"]:663: in function `NewPanel'
[string "@details\core\plugins_statusbar.lua"]:1474: in main chunk

1x Details\gumps\switch-Switch.lua:25: attempt to call method 'SetBackdrop' (a nil value)
[string "@details\gumps\switch-Switch.lua"]:25: in main chunk

1x Details\gumps\janela_principal.lua:1903: attempt to call method 'SetBackdrop' (a nil value)
[string "@details\gumps\janela_principal.lua"]:1903: in main chunk
1x Details\Libs\DF\panel.lua:5017: attempt to call method 'SetBackdrop' (a nil value)
[string "@details\Libs\DF\panel.lua"]:5017: in function `?'
[string "@details\functions\link.lua"]:4890: in main chunk

1x Details\Libs\DF\cooltip.lua:169: attempt to call method 'SetBackdrop' (a nil value)
[string "@details\Libs\DF\cooltip.lua"]:169: in function build_main_frame' [string "@Details\Libs\DF\cooltip.lua"]:290: in function CreateCoolTip'
[string "@details\Libs\DF\cooltip.lua"]:3413: in main chunk

1x ...faceDetails\Libs\NickTag-1.0\NickTag-1.0-12.lua:780: attempt to call method '?' (a nil value)
[string "@details\Libs\NickTag-1.0\NickTag-1.0-12.lua"]:780: in main chunk

1x ...eDetails_ChartViewer\Details_ChartViewer.lua:1334: attempt to index field 'CanShowOrHideButtonEvents' (a nil value)
[string "@Details_ChartViewer\Details_ChartViewer.lua"]:1334: in function <...eDetails_ChartViewer\Details_ChartViewer.lua:1190>
[string "=(tail call)"]: ?

To add a little bit of direction. These are the LUA errors thrown and captured in bug grabber. Hopefully this helps narrow it down a little more than "It does not load".

If you're looking for a quick fix, the Shadowlands Beta version works: https://www.curseforge.com/wow/addons/details-damage-meter-shadowlands-beta

But note:

Please note: after the game launch, this project might not receive updates anymore and you just keep using the Details! version for Retail.

Installing the beta version didn't really work for me. The addon managed to load but nothing was being tracked when getting in combat.

I'm having the exact same issue. I even did a clean reinstall and deleted any trace of details from my interface and WTF folders but the addon fails to load even on first time setup. The reinstall command doesn't work either.
Edit: I'm getting the same errors as mentioned above.

Installing the beta version didn't really work for me. The addon managed to load but nothing was being tracked when getting in combat.

Creating new windows solved that for me. The first window created did not want to display anything at all, but all subsequently created ones worked as normal.

Do not use Twitch to update or the install it. The Details Shadowlands version does work, but it appears to have only worked by doing a manual install of the app after deleting any trace of it from Interface and WTF. It appears that the Twitch client is also seeing it to be a different version even though it's up to date in game. Just ignore doing the update to the addon via Twitch until the author get this resolved.

Is there any reason why the developers are doing this? They clearly have a working version and pre-patch is already on live servers.
Why force us to change to a new addon instead of just pushing the update already existing one?

The dev will probably merge the SL beta one into the live development branch. However there are still bugs in it, so it's a temporary thing until they can get it fully working to push out for the live release.