error: can't place multiple pins assigned to pin location

When you select a configuration scheme, Quartus automatically and transparently assigns the pins. In your case, it's the 4 signals needed by Active Serial configuration.

If you want to access the EPCS for other things than configuration, there's a special feature.

In any case, you don't even need to have those 4 signals in your top level module.

Unless you want to connect custom logic to the AS interface pins -- not sure if it's even possible.