TestMode error msg

Hello,

I activated the plugin and did everything that was required. The link is live and I tested it out by filling out the form and when I clicked “donate” button at the end of the form, I received an error msg:
“(TESTMODE) The merchant login ID or password is invalid or the account is inactive.”

Our authorize.net account is valid and API key and transaction key were successfully created.

I enabled authorize.net. I enabled authorize.net sandbox in the Transaction Mode (I don’t know what that means).

Can you please help?

Thank you.

The page I need help with: [log in to see the link]