Output of op-amp getting as a constant voltage of 500 mV in PROTEUS
You need to add a resistor to the junction of R1/R2 to bias it away from GND.
For a square wave you'd want it (roughly) half of Vcc. Not exactly because the op-amp output does not swing as close to the positive rail as it does to GND.
Eg:



simulate this circuit &ndash; Schematic created using CircuitLab