If the platform notes that uploads are not allowed at this time, it means that the jurisdiction does not allow file uploads outside of review steps in an application.
Some jurisdictions limit the ability to upload files outside of the review steps. When this limitation is in place, an 'Uploads are not allowed at this time' warning is displayed when you attempt to click the Upload button.
The best approach is to reach out directly to the jurisdiction staff to request the ability to upload your revised plan. Staff should assign the Update Application/Update Plan Set task to you in order to allow file uploads again.