what is loading error in op amp?
The Op-Amp can supply a maximum current - and we know that (using Ohms Law) the voltage is a product of the current and the resistance. If the maximum input voltage is, say, 6V, then your output will saturate at 6V if its not linear. To test this, lets forcibly create a large differential to saturate it.

However, say your maximum output current is 50mA, you cannot supply more than that. So for a load of 100 Ohms, you can supply only 50ma - which is (50mA * 100Ohm = 5V). This presents a loading error. Now, for a larger load of 200 Ohms, you can supply the full voltage and you won't saturate at the current - you can provide 6V, and the current would be (6V / 200Ohm = 30mA).

The maximum current will depend on the specific amplifier, and it could be found on the datasheet.