What Do IRS Codes 570/971 Mean? Still No $2,500 Refund After 2 Months & Now Received Letter From Austin TX Office

Filed my taxes in January and still nothing. My transcript shows codes 570 and 971 from last week. Anyone know what this means or how long until I get my refund? Starting to worry since its been over 2 months now. I just received a letter from the IRS today. The envelope shows: U.S. Department of the Treasury Department of the Treasury PRESORTED Internal Revenue Service FIRST-CLASS MAIL Austin TX 73301-0003 Postage and Fees Paid Internal Revenue Service PERMIT NO. G-48 Official Business Penalty for Private Use $300 I haven't opened it yet, but I'm nervous since it coincides with those transcript codes appearing. Does getting mail from the Austin, TX IRS office usually mean there's an issue with my return? I filed electronically through TurboTax and was expecting a refund of around $2,500. The fact that I'm getting this letter after those codes appeared on my transcript has me even more concerned. Has anyone had experience with receiving letters from this particular IRS office in Austin? What could they be sending me?