Issue:

If I understand you correctly, you want to highlight (by highlight, I assume you mean changing the background color) certain sentences if they are present in your document.

Solution:

• For each sentence, use Body.findText(sentence) to find its first occurrence in the document.
• Retrieve the start and end offset of the resulting RangeElement.
• Check whether the corresponding element can be edited as text and obtain a text version of it (see editAsText()).
• Change the background color using setBackgroundColor.

Code sample:

const sentences = ["First sentence", "Second"]; // Your sentences

function highlightSentences() {
  const doc = DocumentApp.getActiveDocument();
  const body = doc.getBody();
  sentences.forEach(sentence => {
    const rangeElement = body.findText(sentence);
    if (rangeElement) {
      const startOffset = rangeElement.getStartOffset();
      const endOffset = rangeElement.getEndOffsetInclusive();
      const element = rangeElement.getElement();
      if (element.editAsText) {
        const textElement = element.editAsText();
        textElement.setBackgroundColor(startOffset, endOffset, "#fcfc03");
      }
    }
  });
}