At the opening of Financial Year, the amounts of the Total Assets should correspond to the Total Liabilities, otherwise it is not possible to have a correct balancing.
If you are shifting from another software to Book Keeper, make sure you have copied all the balances properly, may be amount is incorrect or you forgot to enter.
So, please edit the accounts and make sure that the balances of all accounts are properly entered.
A video explaining how to resolve this is given below. Please note that this video is for Tally, you can follow the same steps in Book Keeper too.