Hi Zachary,
If you're trying to register a map from ArcMap (e.g., a .mxd document) with your ArcGIS Online organization, it won't work. That is not a supported workflow. You need to publish your map document as a map service first, then it can work within ArcGIS Online. You can publish directly to ArcGIS Online and create a hosted feature service, or publish to an ArcGIS Server site (which you would have in your organization).
FYI, How to publish a service from ArcMap or ArcCatalog—Documentation | ArcGIS Enterprise
As Paul said, if you could please clarify what you want to do, then we can help further.
Hope this helps,
