So, from what I can tell from your previous question, you've caught this Uxie in Pokemon Go. Legendary and Mythical Pokemon have this restriction solely if they were from these games, likely because it's possible to get multiple of these Pokemon on different saves beyond trading and transferring up. There are two workarounds to this that are possible:
- Transfer in an Uxie from a different game into Violet - Other games don't have this restriction, and while you can't catch Uxie in the wild in the Gen 9 titles, you can in SwSh Dynamax Adventures, BDSP, and PLA. Assuming you have any of those titles, you can transfer an Uxie from those games into your Violet, and then bringing them in through Home should be made possible. You can also get an Uxie from the Home GTS instead, but that's not all too reliable.
- Trading between Home accounts - If you set up two Home accounts on your switch and then trade the Shiny Uxie between your accounts (on different mobile devices, of course), you can transfer in the Uxie on the account that receives it through trading. You can also trade back and forth between a friend, if you don't have two devices available.
