try hover over it and press L
but rightclick also have the option in the toggle section

Set up all the tiles first, then hold left click and drag a box around them which should give them all a yellow outline. Then press "L" on the keyboard to lock them.

Now place the obstacles and doors. Hold the cursor over each object and press "L" to lock it, there is no need to click the object. Alternatively you can hold Ctrl+LeftClick to select all objects and lock them with one press of "L".

When you want to open a door mouse over it and press "L" to unlock, then "F" to flip it, then "L" to lock it again. You may want to just remove the door tile as sometimes it can make characters wobble and fall over if the alignment of everything isn't perfect.

Assuming you're the host of the game, you should have all permissions. Double check your Options->Permissions has "Locking" check box.

Hmz, I just found out that I somehow collapsed the option to lock. My bad. Thanks for your help.