Legacy Form Fields Lock Button missing in 365

[CarolAnn143]

I am trying to lock Form Fields for use and cannot find the Lock button.

Does anyone know where the Lock button is or how I can find it?

[#2]

In either File > Options > Customize Ribbon or File > Options > Quick Access Toolbar, set the "Choose commands from" dropdown to either "All Commands" or "Commands Not in the Ribbon". Then you'll find the Lock command in the list, after List Commands and before Lock Fields.

[#3]

@Jay FreedmanMany thanks.