I encountered the following errors when trying to either 1) convert money (error trigerred just after clicking on convert money), 2) making the currency account my primary account.
After some research and being unable to contact PayPal (what a warrior journey it is to get anywhere near a contact number or help on PayPal), I found at that all that is require is to click on "add currency" and add the currency which you want to convert. Even though you visually see that currency displayedas one of your account for some reason doesnt acknowledge it until you actually add that same currency.
Once you have added the currency in question, you can then select convert currency and proceed with your request without the error.
Hope this helps as this was rather frustrating for me.