If you are using Simple Delivery you'll need to opt out of this, then you'll be able to select Royal Mail.
When creating a new listing here's how to opt out:
Click on Custom Postage option, this is next to the simple delivery option on the listing on the right hand side, and it should allow you to then select your postage options.
The above is when using a PC / laptop sorry I don't use the app but imagine it must be in a similar place.
@tulsi108