The error 'Specified cast is not valid... ' occurs because data for the Upload Journal has been produced using formulas like VLOOKUP. When the data is copied and pasted the blank cells in the Debit and Credit columns have hidden formatting.
There are two ways to resolve this issue:
When copying and pasting the data, first of all paste the data into another worksheet using Paste Special, Values only, then copy and paste the data into the Upload Journal.
With the data in the Upload Journal, in the Debit and Credit columns, enter values into each blank cell, and click off each time. Then remove each entry, which should remove the formatting as well.