How do I post a supplier payment on account if I havent received the invoice yet

I'll guide you through recording an advance payment to your supplier, info-accounting4. 

First, let's consider your VAT reporting methods to HMRC. If you're on the Accrual basis, you can enter a cheque to your supplier and record them to the Creditors/Accounts Payable (A/P) account. This way, it decreases your supplier balance until you're ready to receive the final bill. 

Please keep in mind not to use VAT codes when entering payments made to your supplier. Here's how:      

1. Go to the Plus (+) icon at the upper right. 
2. Select Cheque.
3. Enter the Payee name, Bank Account and Payment Date.
4. In the Amounts are field, select No VAT. 
5. In the Account/Category column, select Creditors. 
6. In the Amount column, enter the Amount paid.
7. Click Save and close. 

Second, you can enter the bill by performing the steps below: 

1. Go to the Plus (+) icon at the upper right. 
2. Select Bill.
3. Enter the necessary information, including the VAT information.
4. Click Save and close.

Lastly, you can now apply the prepayments to the bill. Here's how:

1. Find and open the Bill. 
2. At the upper right corner, click Make payment.
3. In Outstanding Transactions section, tick the Bill you want to associate with the prepayment.
4. Select the payment created in step 1 above in the Credit section. 
5. The program will create a Bill Payment Check if there's a balance due on the Bill. If you don't want to pay the remaining amount, edit the Amount figure. 
6. Click Save and close.

Here's a great resource which you may find helpful: How to record Supplier prepayments for Products or Services.

For future reference, visit the articles listed on this page about managing suppliers and expenses for QBO: Manage Suppliers and Expenses for QuickBooks Online.

Please know that you can always get back to us if you have any other questions or concerns. Have a great day.