If you receive the message "Transaction Split lines to accounts payable must include a supplier on that split line" OR "Transactions to accounts payable must include a supplier", you have assigned a chart of accounts line to an Accounts Payable account and you must assign a Supplier to Accounts Payable.
How to check if you have the account you have selected is an Accounts Payable account:
Step 1: Open your chart of accounts (Lists -> Chart of Accounts or Ctrl + A]
Step 2: Search for the account that you selected in the journal entry
Step 3: Check the Type of account. This is done by looking in the "Type" column in the chart of accounts or by selecting the account and editing the account (Ctrl + E, or right click -> edit)
Step 4: If it is an Accounts Payable, then a Supplier must be assigned to it within the General Journal Entry