Hi D'Ann,
You can Make this Ledger as a Cash Ledger from Chart of Account.
When you go to Chart of Account after Selecting Ledger which Leger you want to make Cash Ledger you will See Cash Account on Left Side of the Chart of Account Window.
Only you have to Tick Cash Account Check Bod and Update.
After doing this System will not give any error as mentioned by you.
Please also check below link.
Hope this help
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Regards::::
Atul Chakraborty