Mechanical relay circuit schematic and PCB. What is wrong here?
D6 is backwards on the schematic.  It is not clear from the PCB decal which end is the cathode.
Consider adding a 10K resistor from Q3 base to GND to assure a rapid and complete turn-off.
Based on typical values for the Vf of an LED and an optocoupler, it looks like there is only 1.5 mA of current through the opto primary side.  Depending on the minim coil current for the relay, this might not be enough to drive Q3 into firm saturation.
Update:  To increase the transistor's collector current, decrease R11.  A safe starting value is to determine from the datasheet the max continuous sink current for the GPIO pin, and size R11 for 1/2 of that.  Next, use the CTR (current transfer ratio) of the optocoupler to find its secondary current.