Hello and welcome to the Community, isabay
When on the flat rate scheme, any purchases in QuickBooks should be entered with the 20% Standard VAT code applied (these will then be calculated on your return at the FRS rate).
Businesses paying a flat rate usually can’t reclaim VAT on purchases (although there are some exceptions for capital assets worth over £2,000). QuickBooks Online doesn’t support the reclaiming of full 20% VAT when reselling capital goods.
Read HMRC’s guideline for flat rate percentages for different types of business. For further inquiries regarding which FRS percentage you should be using, contact HMRC directly.