I'd like to share some insights ion why you're unable to select the post dated checked that you've issued from the suppliers page.
When creating cheques in QuickBooks Online, the system only allows you to select a bank account and not a liability account. The bank account that the money will be taken from is the one that gets debited when the cheque is produced in QBO.
That said, the cheque will be considered as an expense since it's already issued into your vendor. It is how the system recognises money out transactions. This is the reason you're unable to locate the cheque under the Suppliers page. To view the said entry, you can find this it from the Expenses tab.
To record the post-dated cheque, you may create a bank account for PDC. You can also create a journal entry for this transaction. However, I recommend reaching out to your accountant before doing this so you'll be guided accordingly.
Here's a link that you can check out for more details about the process of creating suppliers transaction and the accounts used in QuickBooks Online: