Did you include that size in your application.xml within <icon> as <image120x120>? It would be worth checking your application descriptor XML file and making sure you're including all of the required icon sizes there (it looks like I have 19 different ones in our apps, though I'm not sure if those are all for iOS or if some are Android-only).
And also did you include a 120x120 icon image in your Assets.car file from Xcode, if you're supplying separate images for each size? I believe the newer system of using one image in Xcode to generate Assets.car would do it automatically, though once in a while there's an issue with App Store Connect handling those vs. one where you added all separate images manually.