Section 16(2) of the VAT Act requires you to hold a valid tax invoice before claiming input tax. A bank statement showing the payment, a card slip, or an order confirmation are not tax invoices, and a claim supported only by those is one SARS can reverse on audit.
A full tax invoice has to carry specific things: the words "tax invoice", the supplier's name, address and VAT number, your name and address, a serial number and date, a description of what was supplied, and the amount showing VAT separately. For supplies under R5,000 an abridged invoice is acceptable, which drops the recipient's details but keeps everything else.
Below R50 no tax invoice is required at all, though you still need some record of the expense.
The practical point is that the claim is only as good as the paperwork behind it. If a supplier has not given you a compliant invoice, ask for one while they still remember the transaction. Chasing documents at audit, sometimes years later, is where these claims are actually lost.