Why commercial juice will not set
Updated
What processing removes
Commercial juice has usually been through steps that take out exactly what you need for a set. The pectin is the obvious loss and the acid balance is the one people forget, and neither is restored by adding sugar.
Commercial juice is clarified and heat treated, and both steps cost pectin. What arrives in the carton is a drink, optimized for clarity and shelf life, and the thing that was taken out of it is the thing a jelly is built from.
The route that does work
There is a way to use bought fruit rather than fresh and it is not juice. It preserves the thing juice processing removes.
Extract your own. The guide specifies bringing fruit and water to a boil then simmering until soft, and notes that using the peels and cores adds pectin to the juice during cooking. It also gives a yield benchmark: one pound of fruit should produce at least one cup of clear juice. Extraction is on extracting juice for jelly.
If juice is the only option
Sometimes it is, and the honest version is that it becomes a different recipe with added pectin rather than a substitution into an existing one. Treat it as a new recipe to test rather than an ingredient swap, because the quantities will not carry across.
Then you are making a product with added pectin, and the recipe to follow is the with-pectin one rather than a no-pectin recipe that you hope will set. That is a legitimate product. It is not the same product.