The antonyms for the phrase "putting a lock on" are phrases that refer to the opposite action of securing something. These could include actions like unlocking, opening or unfastening. Other antonyms could include leaving things unlocked, insecure, or vulnerable.
For example, leaving a window unsecured, or leaving something unattended and open, could be antonyms of putting a lock on something. The opposite could also be true, where putting a lock on something is an antonym for taking a lock off something.
Depending on the context or specific scenario, there may be other appropriate antonyms to describe the opposite of putting a lock on something. However, generally speaking, the antonyms will relate to the release or removal of security measures.