FOUND ANSWER: Must get "Homeline" breakers. (Why different breakers? Just to complicate everything and sell another device?)

100-amp Square D subpanel (in garage) from 200-amp main panel other side of wall. (Installed for a later enclosed back porch 240-volt electric baseboard heat system. 40 amps total.)
Spaces available for more breakers.

Original breakers have a lip which inserts down and hooks over the support rails.
Then the breakers "hinge" up and clip onto their busses. Their clips are INSIDE their plastic housings.

I tried to insert a new 20-amp single-lever double-pole 240-volt breaker for 15.4 amp two-burner electric stove top.
The NEW BREAKER has two metal "forks" projecting out of the breaker body at top: | ][ ][ |
One similar metal non-electric "clip" horizontal-orientation at the bottom of the back of the new breaker.
It clips onto the panel back support rails.

THE NEW BREAKER STICKS OUT A LITTLE MORE than the existing circuit breakers!
Did [D] change their breakers and I must find the older style breakers?

Thank you

No useful info here. Panel make/model? Breaker make/model of old new?

I can only say that some panels will accept a certain style of breaker in only a few places in the panel and a certain style everywhere. Also make sure this is a 240 space and not where two 120's on the same leg would go.

Sounds like you have a Square D Homeline panel and trying to install a Square D QO breaker.

Both are entirely different.

As Kiss mentions. Some model numbers of what your dealing with will help.

Thank you, KISS and TKR, for trying.
You are correct!
Homeline 100-amp interior panel: HOM612L 100RB

Breakers have sticker: HOMT [62]
New breaker: QO220CP [61]