If you are using authorize dot net as a payment processor, as of 2019, they are rejecting the security hash code they have had as a security tool

A fix was released here if you can upload the new file through FTP.  Follow the instructions in post # 2 - this is ONLY if you are using authorize dot net SIM

If you are using authorize dot net AIM, you should just be able to go to Modules>Payment>authorize dot net AIM, click to edit and remove the MD5 Hash code.

If you need our assistance in uploading the code for SIM - purchase this and in the comments box put the new signature key.  We MUST have the signature key to be able to apply the fix for you.