Tax isn't calculating in checkout

I haven't been able to get sales tax to calculate in my checkout. 

I linked my website to TaxJar and entered my tax information but it didn't seem to make a difference. The tax won't calculate at checkout. I'm also confused as to why it asks you to enter tax information manually on TaxJar. I thought the whole point of paying for that service was to have taxes calculated automatically.
Do you have to register to collect tax in every state if you want to sell online? From the research I've done you don't have to register with each state unless you're making a certain amount of money.

I don't understand why selling online and setting up an online shop should be this convoluted.

I'd appreciate any input and firsthand experience.

Thanks!

-Seth

I have this exact same question. Have you found any answers?

It's a little confusing and I'm still not sure I understand 100% but if you don't have nexus (a physical or frequent presence) in a state then you can't charge sales tax in that state, though I don't quite understand how that works when it comes time to report taxes..