CORE does not allow reconciling balance sheet accounts that are set as the default System and Tax Accounts in Settings > Accounting. This is because these accounts are internal to CORE and you do not have a second source (like bank or credit card statements) to verify or reconcile such transactions. System accounts are those with account numbers 12000, 12010, 3900 and 2200 and tax accounts are those with account numbers 25100, 25200, 25300 and 25400. The exception to this is the two system-defined accounts (3000- Opening Balance Equity and 2100- Payroll Liabilities), which are available for reconciliation.So, the option to reconcile those account is disabled or made unavailable. However, if you specify your own chart of accounts and set those up as the system and tax accounts, then you can reconcile them. Check CORE Help Center for more on Reconciliations.