I bought a course in installments. E.g. if I bought SOK, on paper I "got access" to SOK, but when I got in, I found out half was unreleased. They delayed it many times and 1.8 yrs later it's still unreleased. I've used what exists. I paid 2/3 and they're asking for the rest. Must I pay it in full?

Ustadh AbdulAziz al-Haqqan

This goes back to what you both agreed, because the Muslims are bound by their conditions. If they did not deliver what was agreed, you may ask them to complete it or agree with them on a reduction for what is missing.