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[Error] LightningExperience : Error parsing file: Element {}enableLightningPreviewPref invalid at this location in type LightningExperienceSettings

I was having the same issue, what fixed it for me is specifying that I want the preview release in my scratch org definition file by adding:

"release": "preview"

Definition should be similar to:

{
  "orgName": "Demo company",
  "edition": "Developer",
  "release": "preview",
  "features": ["EnableSetPasswordInApi"],
  "settings": {
    "lightningExperienceSettings": {
      "enableS1DesktopEnabled": true,
      "enableLightningPreviewPref": true
    },
    "mobileSettings": {
      "enableS1EncryptedStoragePref2": false
    }
  }
}