Sometimes a bank account gets added twice: once through Integrations (with a live feed) and again by hand in the chart of accounts. If transactions were then categorized to the chart-of-accounts copy, here's how to clean it up.
Why you can't post to it
Only accounts linked to a real bank or card feed, or a manual account set up in Integrations, show up in the Add transactions dropdown. An account that only exists in the chart of accounts can't be picked there or in a manual journal entry. It also won't appear in Merge Accounts, because it isn't a bank account record.
How to clean it up
Go to Transactions → Bank & Credit Card and find the transactions categorized to the duplicate account.
If a transaction has a green check, unfinalize it first.
Change its Category to the account it belongs in, for example your clearing account, then Finalize. For many at once, select the rows, click Categorize, choose the account and finalize.
Once nothing is left on the duplicate account, go to Accounting → Chart of Accounts, click ⋯ next to it and choose Deactivate.
Recategorizing only changes where the other side of the transaction posts. The bank account, amount and date stay the same, so your reconciliations aren't affected. You don't need to delete anything.
When to contact us
If the balance on the duplicate account came from a historical journal entry, or from merging bank accounts, it can't be cleaned up this way. Message us in chat with the account name, one or two example transactions (date and amount) and a screenshot of one opened up.