The possible reason why your balance is showing n/a is that it's being sorted by other transactions. Let's click again the Date column so you can see your balance back. Also, make sure that running balance is checked as a column in the small gear icon in the register.
Select Accounting from the left pane and click Chart of Accounts.
Locate your Bank account then click View register.
Click Date.
For future use, I'll share with you some other helpful articles about configuring the Register: