Slim line or tandem breakers will only work if your panel accepts them. The info sheet on the panel door should say or check the model number for a "3040" type designation. If they are accepted, they will only fit in the bottom 10 spaces (5 each side).

While the limit for breakers in a residential panel was lifted if your AHJ is not under the new code you may be limited to 42 breakers.

Some boxes were designed to not allow the reduced size breakers to be installed and go over the 42 limit.
A box designed to hold 42 full size breakers is not likely to be listed or accept any half size breakers, and even 30 slot boxes were limited to only 10 half sized (often in the bottom slots only).

If you post the exact manufacturer and model number some of us may recognize what the box can hold.

Thanks, fellows for the info. I didn't have to use the slimeline after all. I looked closer and somebody had left a 30 amp double pole breaker in the box that wasn't hooked to anything. I removed it which allowed space for 2 15 amp single poles and used one of them. Thanks.