I can tell you the working process:
1. Reinstall TT7 & after reinstall do not run TT6
2. Start TT7 select opencart with scratch, do not select any old theme ( all of them are TT6 supported not TT7)
3. Work with your theme , but do not change your cms while working with opencart.
4. After completing your design then click export, hope it will export your theme. If anytime exporting does not work then reinstall TT7 again & follow the rules that I mention above.
I already design one site but still have some bug with this design, however it's not bad at all with this beta TT7. You can take a look with my opencart design https://www.ibazare.com