Bs isn't a 3 ISO Code for Currency I assume it's Bahamas Dollar therefore you must include BSD and tick your "Actualizar precios" as shown in your image to automatically update the currency. And now you'll be able to change the default currency
How to Change the Default Currency:
Note: If invoices already exist in your system in this currency, editing will change the currency of those invoices without adjusting the amounts.
Start by going to Setup > Currencies
The default currency is signified by the lack of a delete icon. On new installations of WHMCS, the default currency is U.S. Dollars (USD).
Click the Edit button.
Update the fields with the information for the currency, including the correct currency codes.
Finally, click Save Changes.
I hope this helps