Hi, This is related to the PayPal account/API key (it’s not an issue within the plugin). With that said, few things to check:
1) PayPal doesn’t support the express checkout API in all countries. So you need to make sure that you are using a PayPal account that supports it. Before switching to a new account, did you verify with PayPal that they support express checkout for the country that the new account is in?
2) If you plug in the API values of the account you were using before, does it work fine?
3) Did you crate a NEW app this this new account and get BOTH the live and sandbox keys?
Make sure to plug in ALL the keys (both the live and sandbox keys). Sometimes users only plug in the sandbox keys then they try to use live mode and that will produce an error.
4) Are you using live mode or sandbox mode? Do you have the “Live Mode” checkbox check in the settings menu of the plugin?
Paypal’s sandbox servers can sometimes have issues. They can take a few days to sort it out and in the meantime you will think that your API keys are wrong. So best to try a transaction in Live mode using a $1 product.
5) Do you have any page or database caching on this site? Does your hosting provider use caching? It is very important that the OLD API key values are not being cached by your server. Contact your hosting provider and request to empty all cache so the new values can take effect for sure.