We had this issue when trying to upload stock items that had only one attribute or more than 3 attributes. The solution we used for the more than 3 attribute was to break the items out further. This one was due to a Shopify limitation, where Acumatica allows you to have a huge number of attributes per template.
The template item had attributes like size, color, connection style, and a checkbox for back brace or no back brace (for a certain type of fall harness). We ended up having to reduce the number of attributes and just broke out the harnesses to two templates, one for the harness with a back brace and one for the harness without a back brace.
For items with only one attribute, acumatica won’t support it. They require at least two (something I don’t understand but that’s how it works to the best of my knowledge). We created dummy variables for these and it solved our problem. Your dummy variable can be something trivial like color and it can be the same for every variant, but it has to be there. I know there are other threads on dummy variables in template items if you look for those! Hopefully this is helpful!