What is hydraulic system stick-slip effect? when it will occurred during 120 MT load lifting by continuous casting machine's ladle turret?

Stick-slip is a sometimes what is perceived as jerky or irregular motion that can occur when a hydraulic actuator, such as a cylinder or motor, alternates between periods of sticking (static friction) and slipping (overcoming kinetic motion).  It can occur when the machine or motion is moving between static friction and dynamic friction in the system’s moving parts.  

In an ideal hydraulic system, motion would be smooth and proportional to the flow of hydraulic fluid. However, if frictional forces at seals, bearings, or sliding surfaces exceed the hydraulic force momentarily, the component “sticks.” When the hydraulic pressure builds high enough to overcome this friction, the component “slips” suddenly. This cycle repeats, producing a chattering or jerking motion instead of continuous movement.

In a turret application it is where the stiction in the turret gears and bearing begins to bind causing it to stick, and then the resistive pressure builds beyond the load where it “slips” free again.  Some typical causes could be: excessive seal drag or poor lubrication between the bearing seal and turret gear; low rotational speed making small friction differences more noticeable; uneven loading or frictional forces within the turret, where in some instances this could be alignment with the rotational gear drive mechanism.

During the 120 metric ton load situation, this may need to be evaluated with the system design parameters to determine if it is near a possible overlap condition.  This condition could also be observed if the operational load is near a circuit/system relief setting.  

Turret load lifting hydraulic system pressure is 200 bar generated by variable displacement piston pump and system is incorporate with bladder type accumulator, but currently accumulator was inactive. So is there any reason for metallic knocking sound heard due to accumulator not functional and isolated from system?

Not knowing the full machine parameters or the system schematic, it is hard to say for sure the root cause of the noise you are referencing. Many factors within the turret system could be the root cause or source of the noise. It may be best to refer to the machine's service manual or the manufacturer for specific machine settings and service/repair recommendations.

Keep in mind that one of the uses for an accumulator is to absorb shock and provide cushion to a dynamic load or surge.  I would suggest referring to the accumulator topics outlined in both the Hydraulic Specialist (HS) and the Mobile Hydraulic Mechanic (MHM) study manuals.  Accumulators are covered in detail within the MHM study manual under outcomes 1.2.6, 3.8.1, 3.8.2, 3.8.3, and 4.3.1. These can be found on the website at the study manuals link and can be downloaded for free as a member benefit for any active IFPS members.  https://www.ifps.org/study-manuals

You mentioned that the accumulator was "inactive". Please keep in mind the safety tip called out within the MHM. "Safety Tip: Extra caution is required when troubleshooting systems that utilize accumulators. Since the energy stored in the accumulator is often critical to the proper operation of the system, it may not be practical to operate with the accumulator isolated and discharged. However, when making adjustments or working around any machinery with accumulators, proper lock-out/tag-out procedures MUST be followed to ensure a safe working environment." Also, "If the precharge pressure is low, system operation will be sluggish if the accumulator is used for additional flow rate. Low precharge in accumulators used for surge protection will allow a system with high system shocks to knock and make noise."