Forget about the nwell bulk for now, u might be getting the error because u placed two psub bulk on a single layer.

if you want to clean it
If u are using TSMC u will be having a layer in the LSW by name psub2 and place a rectangle around anyone block either switch or Opamp.

If u are using chrt u will be having a layer by name sxcut do the same as explained.

Check whether this works.

This psub2/sxcut isolates your vss from gnd or viceversa.

make sure all the vss/gnd nets come under the psub2/sxcut layer.

Reply back if it works.

If that works then the actual process is to place a dnwell, what ever explanation i gave u is used only for the cleaning LVS.

Hope this solves ur problem.