the prepayments journal page will tell you the issues preventing you from posting your journal.
The most common issue is a bill which has been posted to the prepayment account where no expense account has been identified in ScaleXP.
See How to automate your prepaid expense journal: review and post the prepayments journal
Related Articles
How to Reconcile Your Prepayments Balance with Xero or QuickBooks
How to Automate Your Prepaid Expense Journal: Review and Post the Prepayments Journal
How Should I Code My Bills? (to expense or balance sheet)
How to Automate Prepaid Expenses: Setting up Prepaid Expenses in ScaleXP
How to Change the Review Status of a Supplier or Bill on Prepaid Expenses Pages