Why can't I exclude partially matched transactions?

Thank you for posting, @ambrose1.

Can you tell us more about this partially matched transaction? 

Please be advised that only transactions in the For Review tab can be excluded. If you have matched or added this transaction, then you must Undo this transaction to get this back to your For Review tab.

Check this article for more information: Exclude expenses from downloaded bank transactions. Also the following article on how to match and categorize bank transactions in QuickBooks Online. 

I'm always here if you need help with your transactions. Have a wonderful day!