A family of Microsoft relational database management systems designed for ease of use.
First, the problem is not SharePoint, per se. However, depending on what you mean by "...moved to sharepoint" there could be a problem related to that.
So, how DID you move to SharePoint? What does that mean? Did you export your Access tables to SharePoint lists using the Wizard in the ribbon? If so, did the newly created SharePoint list replace the previous native Access tables in the accdb which is the Front End?
Assuming that is how you did it, you can turn your attention to the form's data properties, as shown here.
Experiment with the various "Allow..." and "Data Entry" properties to determine if that is the problem here.