Ash here from the Shopify team. Welcome to the Shopify Community!
Thank you for posting a screenshot with your question to help show the error you're getting, that's a great help! I'm happy to assist with this. In order to help out to the fullest, I'll need to get some more information from you. Could you please reply back to this post and provide the following information?
I look forward to hearing back so we can continue to work through this together!
Ash here again. Thank you for getting back to me with this update! It's great to hear you're getting assistance from Authorize.net to set up your account with them. Feel free to keep me posted once you know for sure if that was the problem. If not, I'll be happy to continue assisting any way I can.