Designing a compensator - Control system - Missing value
I need to know its value

You can estimate it from the data sheet: -

So, it has an open voltage of typically 5.5 volts and it has a shorting current of 1.2 mA. This means it typically has an internal pull-up resistor of 5.5/0.0012 = 4583 ohms.
This is a typical value and as you can see it might be 5.5/0.0022 (which equals 2500 ohms) or 6/0.0012 = 5000 ohms.
I'd probably assume it was around 3500 ohms.