I am actually trying to design a basic mux with Openlane and this is the error i am facing in step 7 of the openlane design flow , if you could help me out with it.

The error says "Utilization exceeds 100%". This means that you need to increase your floor plan area by increase the two last values in ::env(DIE_AREA) in config.tcl. You can also use the relative floorplan:

set ::env(FP_SIZING) relative                                                                                                                                 
set ::env(FP_CORE_UTIL) 20

Thank you so much , that rectified that paticular area , im getting another error in CTS , can I also put forth that to you /

I think maybe you can enlarge your design more. There are some overlaps that cannot be resolved.

can u send me the part of config.tcl specifying ::env(DIE_AREA)

You can find out all the configuration of Openlane here: https://github.com/efabless/OpenLane/tree/master/configuration. If you do not set them, they will get the default value. In your case, maybe you can try to comment out "set ::env(DIE_AREA)" and change FP_SIZING to relative and add FP_CORE_UTIL as follows:

set ::env(FP_SIZING) relative                                                                                                                                 
set ::env(FP_CORE_UTIL) 20

thank you so much

Which problem do you have? You can compile the gate-level netlist, the library and the testbench as you compile the RTL. Then you can simulate it and dump the vcd file.

how do i compile the gate level netlist??

It is similar to compiling the verilog RTL, you replace your verilog RTL with the verilog gate-level netlist and the library.

oh ohk , but particularly do you have an idea on how to simulate it on ngspice. ?? , basically openlane has created a .spice file for me , im just not able to use it in ngspice

Simulation using spice netlist will take a lot of time. But you can do it. If your design is purely digital it is not recommended. You will need to create a testbench for spice netlist and include the spice library for it.