[SalesForce] Lightning component flow custom validation error message

I FIGURED IT OUT

I couldn't use that button in lightning because it was passing in field parameters. Instead, I created a custom action on Contact that launched my flow. I made sure my Contact.Id variable was named "recordId," because SF requires this in order to pass in the ID while launching the flow (documentation). Works perfectly now.

You will also need to dispatch the FlowAttributeChangeEvent telling flow the attribute value has changed like this

 handleChange(event) {
    let length = event.target.value.length;
    const attributeChangeEvent = new FlowAttributeChangeEvent('fieldValue', event.target.value);
        this.dispatchEvent(attributeChangeEvent);  


    // console.log(`length: ${length}`);
    if (length > this.characterLimit) {
      this.errorMessage = `Your answer must be less than 255 characters. Your character counts is ${length}`;

    } else if (
      (length <= this.characterLimit) &
      (length > this.characterLimit - 25)
    ) {
      let left = this.characterLimit - length;
      this.errorMessage = `You have ${left} of ${this.characterLimit} characters remaining`;
      this.fieldValue = event.target.value;

    } else {
      this.errorMessage = "";
      this.fieldValue = event.target.value;

    }
  }