This happen because you defined values as empty object without keys. So error make sense: this.values[key] is undefined when you trying to access to.

All you need is add check if object property exist and set default value if not:

type Keys = 'a' | 'b' | 'c';
type IV = {
  [key in Keys]?: number;
}
class User {
  private values: IV = {};

  addCount(key: Keys, count: number) {
    if (typeof this.values[key] === 'number') {
      this.values[key] = (this.values[key] || 0) + 1;
    }
  }
}

I think type Keys = 'a' | 'b' | 'c'; the first line is wrong. here you should provide a data type not a value.