What determines Charge Controller PV input limits?
What you're missing is the fact that the input specification is telling you how much power the charge controller needs from the PV array. It is not a limit that you must enforce. On the contrary, it means that your array needs to be able to supply at least that much power in order to max out the charger.

The charge controller is designed to deliver up to 50 A to either a 12-volt or a 24-volt battery pack. At 12 volts (nominal), that represents an output power of 13.7 V &times; 50 A = 685 W. Since they're telling you that it requires 750 W of input power, you can infer that the charger's overall conversion efficiency is about 685 W / 750 W = 91.3 %.

If your PV array voltage is 68 V, then you need to be able to deliver 750 W / 68 V = 11 A to the charger in order to run it at its full capacity. Since your array can actually deliver 15 A, you will not be able to use the full output of your array. In other words, your array has excess capacity relative to what the charger can actually use.

On the other hand, if your battery pack is 24 V, then the charger needs 1500 W, and your array would have to deliver 22 A to run it at full capacity. Since your array can only deliver 15 A, the charger will charge the battery at a maximum of about

$$ \frac{15 A \cdot 68 V}{27.4 V} \cdot 91\% = 34 A$$

This is fine; it just means that your charger has some excess capacity relative to your PV array.