Metal detector circuit description
The left opamp creates a Wein Bridge oscillator at about 78 kHz. L1 is probably the detector coil, whoever designed this should be able to tell you how many turns of wire and what diameter the coil should be.
I am assuming that when metal is put near the coil the resonant frequency will change to something near the oscillator frequency. When the sine input to L1/C4 hits the resonance frequency, the signal will be amplified. https://en.wikipedia.org/wiki/LC_circuit
The right opamp amplifies the signal further and the diode and cap on the right are a peak detector. You will need to connect something with high impedance to the right side, maybe a voltmeter.
The Wein Bridge oscillator is difficult to simulate (from prior experience), but the middle part is easy. L1 needs to be about 14 uH for a resonance of 78 kHz.
This is a really poor, incomplete metal detector. The sensitivity will be terrible. You should find a better circuit.
Just to get the Wein Bridge oscillator to work is a challenge. It needs a non-linear element to set the gain, read the Wikipedia article.  https://en.wikipedia.org/wiki/Wien_bridge_oscillator



simulate this circuit &ndash; Schematic created using CircuitLab