Hi, @Frank38 !
Katy here from Shopify. I hope all is well with you.
You should be able to import a theme when using any plan that includes the online store feature. This is available by default on the free trial, and after your trial has finished it would then be from the Basic plan upwards. The only plan where this feature would not be available is when using the Lite plan.
I am sorry to hear that you seem to be having difficulty with this. Being on the Basic plan it should indeed be working for you. There are a couple of things which we can cross-check to help:
If your theme is compatible, I would then ensure that your browser is fully up to date before attempting to add this to the theme section of your admin. Do also ensure to upload this as a .zip file. We have some info on this here.
I hope this helps - do let me know if you have any questions at all.