In Ubuntu, Linux Mint, and other desktops, the main window manager already uses the Alt+Click, so it captures that shortcut rather than OBS.
Solution 1 - Permanent - Modify the existing handler to allow OBS to receive the Alt-Click
You need to disable this behavior or change the modifier key to a different one. For Ubuntu, you may need to install dconf-editor
as noted in this answer. For Linux Mint, this is a built-in option within the Windows preference panel.
Solution 2 - Temporary - Press the Windows key along with Alt
Alternatively, you can make OBS work without changing anything at all. Simply press Windows+Alt+Click to crop the scene sources. That stops the desktop manager from performing its window drag shortcut, and OBS accepts or ignores the extra Windows keypress and executes its crop as expected.