can't equip slim arms on mobile

i have been trying to equip slim arms for my custom skin for the past 2 days. but when i select them it doesn’t show equip on the right side, it seems i can’t even select it (when i tap on it it does’t do anything, it does outline in blue and not green like the other options). I tried restarting, reinstalled minecraft, restarted my phone multiple times. but nothing seems to work. other customisations seem to work just fine it is just arm size for me.