I've never seen that message in Sage 50 Canadian. 

Are you using a separate 'Cash and Cheques On Hand' account to record receipts, which are moved to the bank account when you prepare Deposit Slips?

If you record every receipt directly into the bank account, then use the Deposit Slip record taking the individual items back out and putting the total in, it's difficult to reconcile because of the additional transactions.  

Possibly one of the deposit items in deposit was later reversed after the deposit was posted, and that's what is meant by '...deposit slip details that are currently not included in the statement transaction list'.

Can you open Deposit 30 to see the detail?  Sage 50 doesn't allow adjusting deposits, so errors are difficult to correct.