How do I track down the cause of 'Undefined symbols for architecture x86_64'?

This is a little embarrassing, but I'm getting several self-inflicted errors after, I believe, fat-fingering my settings. I was working away and hit a strange combination of keys and the next thing I knew I was in my project settings. I didn't think much of it and so I went on working and the next time I tried to build my project, I started getting `Undefined symbols for architecture x86_64`, `ld: symbol(s) not found for architecture x86_64`, and `linker command failed with exit code 1 (use -v to see invocation)` errors:

Undefined symbols for architecture x86_64:
  "One_Rep_Max.LiftEvent.weightUnit.materializeForSet : One_Rep_Max.WeightUnit", referenced from:
      protocol witness for One_Rep_Max.LiftEventProtocol.weightUnit.materializeForSet : One_Rep_Max.WeightUnit in conformance One_Rep_Max.LiftEvent : One_Rep_Max.LiftEventProtocol in One_Rep_Max in LiftEvent.o
  "One_Rep_Max.LiftEvent.lift.materializeForSet : One_Rep_Max.Lift", referenced from:
      protocol witness for One_Rep_Max.LiftEventProtocol.lift.materializeForSet : One_Rep_Max.Lift in conformance One_Rep_Max.LiftEvent : One_Rep_Max.LiftEventProtocol in One_Rep_Max in LiftEvent.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Here's the change I made in my project that seems to be related because I added it to the file 'One_Rep_Max.LiftEvent':

protocol LiftEventProtocol {
    var liftName: String { get }
    var formulaName: String { get }
    var weightUnitName: String { get }
    var liftWeight: Double { get }
    var repetitions: NSNumber? { get }
    var maxWeight: Double { get }
    var lift: Lift { get set }
    var weightUnit: WeightUnit { get set }
    var liftEventDidChange: ((LiftEventProtocol) -> ())? { get set }
    var context: NSManagedObjectContext { get }
   
    func calculateOneRepMax()
}

In my search for answers I've found a lot of people who have similar problems are using third-party libraries but I'm not using any in my project. I've tried many of the suggested remedies like:

1. cleaned my project

2. deleted my Derived Data folder

3. Made sure all of my project files had their Target Membership set to my project

4. I **would** try to `use -v to see invocation` but I'm a noob and I don't know where I would do that

5. Checked many of my settings as suggested in a lot of the threads I've found

I don't want to try other things blindly because I'm afraid I'll make it worse. From the screenshot above, can anybody help me decipher what it says and give me a little guidance on what to try next?