As you’re on flat rate VAT (FRV), you need to check the box ‘Allow VAT to be reclaimed on this purchase’ on the purchase invoice directly:
This will enable the VAT to be added to the purchase invoice and then reversed charged, make sure to enter the rate of VAT that would have been charged as if the purchase was made in the UK i.e. 20%:
If you need any further help, please let us know.