I just ran into this weird issue, that I cannot uncomment my code in a script file online on Matlab.
I can do command ⌘ + / to comment my code, but I cannot do the inverse to uncomment it. I've tried to use command ⌘ + T, which opens a new tab, command ⌘ + / again, which keeps commenting my code, and command ⌘ + R, which restarts the kernel.
The uncomment command conflicts with the open new tab command. The latter is prioritised because MATLAB online runs in the browser.
Moreover, shortcut customisation is not available in MATLAB Online.
So your only chance to make the uncomment shortcut work is to change the open new tab shortcut in the browser.
EDIT: For an alternative solution that does not involve shortcuts, please check the answer by Harshal.