I've installed gnome-shell-extension-multi-monitors 19-1 on ubuntu 20.04 and running gnome-shell 3.36.4:
The extension is available within gnome-tweaks and can be enabled. But when logging in it crashes: gnome-tweaks shows "Error loading extension" and the following error shows up in the syslog:
Dec 14 07:27:27 eso12919 gnome-shell[3569]: JS WARNING: [/usr/share/gnome-shell/extensions/multi-monitors-add-on@spin83/mmpanel.js 402]: reference to undefined property "_onButtonPress"
Dec 14 07:27:27 eso12919 gnome-shell[3569]: JS ERROR: Extension multi-monitors-add-on@spin83: TypeError: this._onButtonPress is undefined#012_init@/usr/share/gnome-shell/extensions/multi-monitors-add-on@spin83/mmpanel.js:402:4#012_pushPanel@/usr/share/gnome-shell/extensions/multi-monitors-add-on@spin83/mmlayout.js:220:15#012_monitorsChanged@/usr/share/gnome-shell/extensions/multi-monitors-add-on@spin83/mmlayout.js:199:11#012showPanel@/usr/share/gnome-shell/extensions/multi-monitors-add-on@spin83/mmlayout.js:87:10#012enable@/usr/share/gnome-shell/extensions/multi-monitors-add-on@spin83/extension.js:209:24#012wrapper@resource:///org/gnome/gjs/modules/script/_legacy.js:82:27#012enable@/usr/share/gnome-shell/extensions/multi-monitors-add-on@spin83/extension.js:331:21#012_callExtensionEnable@resource:///org/gnome/shell/ui/extensionSystem.js:167:32#012loadExtension@resource:///org/gnome/shell/ui/extensionSystem.js:349:26#012_loadExtensions/<@resource:///org/gnome/shell/ui/extensionSystem.js:599:18#012collectFromDatadirs@resource:///org/gnome/shell/misc/fileUtils.js:27:17#012_loadExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:574:19#012_enableAllExtensions@resource:///org/gnome/shell/ui/extensionSystem.js:608:18#012_sessionUpdated@resource:///org/gnome/shell/ui/extensionSystem.js:639:18#012init@resource:///org/gnome/shell/ui/extensionSystem.js:56:14#012_initializeUI@resource:///org/gnome/shell/ui/main.js:257:22#012start@resource:///org/gnome/shell/ui/main.js:146:5#012@<main>:1:47
I've checked https://extensions.gnome.org/extension/921/multi-monitors-add-on/ and it seems that the version 19 of the extension is not compatible with 3.36.4. It seems like you need at least version 20 of gnome-shell-extension-multi-monitors. So I guess an update is necessary...
