The short version is that there's simply no substantive state's rights aspect to the American Civil War. At all. Anyone telling you this is lying for dead white supremacists. If they are doing this knowingly, which is likely, then we can remove the qualifier of "dead" from the prior sentence and ad that they are seeking to recruit you to their cause. Pointing out that the only state's right the treasonous major enslaving states were committed to was to keeping slavery, a system of torture and rape for profit, going in perpetuity are saying not that there is a state's rights aspect we might meaningfully discuss, but rather calling out the lie.

I discussed this a fair bit in a recent answer to a similar question, but can always expand further.

It was absolutely about slavery.

The core underlying issue leading to the outbreak of war was slavery, and, although individual motivations for those fighting for the Confederacy may have been more nuanced, the secessionists themselves - particularly those states that took part in the first wave of secession - could not have been more clear that slavery was at the heart of the issue. Declarations of secession (such as that of South Carolina) often made claims of other grievances that are often put forward as "states' rights" issues, but the only issues ever discussed in any detail in declarations such as South Carolina's were the existence and preservation of slavery. The closest you really see to "states' rights" issues discussed as part of secession declarations is references to the Fugitive Slave Act, particularly the non-enforcement of such in many non-slave states. That's not to say that other issues didn't exist that contributed to the outbreak of the war, but no factor held anything close to the significance as the issue of slavery.