If the Android or iOS mobile app displays a modal showing "Unable to create account" it's because you have already created an Unstoppable Domains account via the web app, but previously used the Connect Wallet option in the web app.
To manage domains in the mobile app that you have already claimed to a wallet created with the web app there are 2 options:
1. Import the wallet into the mobile app (with recovery phrase or private key).
2. Create a vault in the mobile app and transfer the domain to the mobile vault address, via the web app.
With option 2 you would need to add the new mobile vault to the web app if you also want to manage your domains via the web app.