'Cash purchases' are any payments you make to suppliers of stock with whom you do not have an account - so you have to pay for the goods when you buy them. For example, you might buy some items from a local cash and carry, or from an online supplier.
Cash purchases only include items that you buy for use in providing pharmacy services and sales. They don't include things like tea and coffee bought for use at work, or items such as business stationery. Don't include things like this here. They're covered elsewhere in the cash flow.
Make an estimate of the amount you are likely to spend on cash purchases and enter this in the months when the expenditure will be incurred.