Question: Why am I getting this error popup, “This GL Entry will cause an out of balance and cannot be completed. Please modify your edits and try again.”
Bank account GL's are system maintained GLs. Since you're creating a GL Entry to a bank account, it won't create the bank register item, so the system is trying to prevent you from having an out of balance down the line by giving you that error.
Instead of a GL Entry you'll do a post deposit, post expense, or transfer funds on the reconciliation screen. That way it'll create the bank register item and you'll be able to reconcile.