This issue with making stickers appears to be fairly common, but not necessarily universal. I have tried all the suggestions posted here and elsewhere. To date, nothing has worked. However, I can make and use stickers from an iPad Air 4 and an iPad Mini 5. I can also use stickers made on my iPads with Messages on my MacBook Air M1 (2020). My wife and I have new, identically appointed iPhone 15 Pro Max devices. The irony is that my iPhone has issues with making stickers, but my wife's iPhone does not. After carefully comparing both iPhones' settings, the only difference I can find is she sets her Messages to delete after 30 days. Mine was set to Never delete. Now, I am running an experiment where my Messages are set to delete after 30 days. I have 187 Messages that show as available, but they are not. I guess they were stuck in iCloud from the past. I am running under the hypothesis that one or more of these 187 mystery iCloud Messages has an errant sticker in it somewhere and/or some other bug interfering with proper sticker operations. I am still not through this 30 day experiment, but my mystery Message count is going down. I am not going to change any other settings or any other troubleshooting at this point until the experiment is over. At the end of the experiment I will post my results, positive or negative.