Sorry to be so late in answering the original post.
Welcome to the wonderful world of conventional undercarriages.
The Chipmunk doesn't have a lockable tailwheel because it doesn't need one; only needs proper handling.
The reason you are finding it challenging is that the BASIC flying skills required were not sufficiently developed by the previous types you have flown.
All you need to master the aircraft is a change in mindset.
Two common questions asked by new taildragger pilots are 1, which way does it swing? and 2, how much rudder do you need?
The correct answers are, 1 It doesn't; not when I fly it, and 2, as much as it takes.
These answers may not be helpful initially but when you consider that no two take-offs or landings are the same but they are the only answers which can accurately fulfill all the possible situations and show your confidence.
The Chipmunk is a lovely aeroplane, by the way.