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no put it at the very top level, in your case the schematic of the ring oscillator. if you want to test just one inverter, create a new schematic with just the inverter symbol with the vdd-vdc-gnd circuit and simulate. or replace vdd with vdc=3.3 (or whatever your technology uses) in the schematic of inverter.
why are there output pins? is this the top level or there is this the schematic of a symbol? make sure all pmos source is tied to vdd (from basic library, symbol) and in top level you have the vdd-vdc-gnd circuit. if you are really struggling, tie all pmos to vdc=1.8. does the opamp give output? Vid seems to be zero.