Of extreme importance is the fact that the Import/Export missions can only be done in Public sessions. If you want to play solo but still do these missions, you can force a solo public session through the following steps:
1. Alt-Tab
2. Open Task Manager
3. Click 'More Details'
4. Click 'Perfomance'
5. Click 'Open Resource Monitor'
6. Under the CPU tab, find GTA5.exe
7. Right click GTA5.exe
8. Click 'Suspend Process'
9. Wait about 7 seconds
10. Click 'Resume Process'
11. Alt-Tab back into GTA Online