Entering the purchase invoice for a second-hand device as exempted from VAT

I bought a second-hand device from an official Dutch seller (with a vAT number).
On their sale invoice, the amounts including and excluding vAT are the same (at the bottom it states that the margin scheme applies).
When I enter the invoice in dexxter (I am exempted from VAT as a secondary occupation), it says that I am not entitled to VAT deduction.
Should I just ignore this message and leave the VAT inc. and VAT excl. boxes the same?

Or should I deduct 21% from the amount I paid and enter this in the ‘VAT excl.’ box?