After you e-file a return or extension, the TaxAct Online program will not allow you to import any data into the return. This prevents overwriting the information you filed to the IRS, because importing overwrites return information.
If you e-file an extension before importing your data from last year, you will not be able to import any data into the return. If you want to import your data from last year, you will need to create a new return, and import the data into the new return.