Basically a credit card reconciliation is just trying to match each transaction on the statement with a transaction in your credit card liability account.  When you see an entry on the statement and see a transaction on screen, you put a check mark on screen and on paper.  Anything without a checkmark on paper needs to be posted to the books.  Anything without a checkmark on screen, should be checked to make sure it is a real entry and see if it is possible that it would not clear the account before the statement end date.

When you can't figure out something, the first thing to check is the date assumption.  Are the Start and End dates on the statement the same as what you entered in the reconciliation (check the exact fields, this has nothing to do with the Reconciliation date on screen.  The wrong dates can hide entries you need to reconcile (but then you would have entries on the statement unchecked also).

Check the start and ending balances on the statement to make sure they are also on screen properly.  Wrong end balances can throw off your Unresolved Amount.

Uncheck Do not Show Corrections.

A Negative Unresolved means you checked off too many charges on the card vs. what is on the statement, or you have not checked off all the payments yet.

Assuming you mean that the statement end date is the 20th of the month, that just means that some entries from last month should be on the statement if purchases or payments were made during that time period last month.

Now review all the entries again and see if anything is not checked off properly.

My assumptions are that this is not your first reconciliation in Sage 50 for this account and that all entries have been posted.  It is quick likely that some entries have not been posted or some have not been posted during the period indicated by the statement dates.  eg. if a statement ends Jun 19, you cannot reconcile an entry dated in Sage 50 on Jun 20 if you use Jun 19 as the statement end date.

BTW, I never allow the module to post an entry.  Find out what is missing first.