The only options you have to “uncheck” that box would be to either:
a) restore a backup file dated before the date that you closed that account in Quicken, or
b) add a new account in Quicken (to replace the one you’ve closed) and then “Move” all the transactions from the old “closed account” register into the new account register. After successfully setting up the new account and doing the transfer, you could then delete the old “closed” account.