Form 8453 says you can mail statement with equivalent information. But generally you shouldn't need to do this unless you have some large volume of adjustments you can't import and don't want to input.
If you already entered sales which have adjustments (wash sales, AMD etc) or non covered securities on Form 8949 which will reflect on Schedule D lines 1b/8b onwards, and any Sales Summary entries do not have adjustment codes which will reflect on Schedule D line 1a/8a, then no additional mailing should be needed. If TT is telling you to do this double check the 1099s, unless it's some issue with TT prompting needless mailings to IRS.
Desktop version provides a prompt to confirm whether mailing is needed, and you can see it on 1099-B worksheet forms. I had 30 detailed entries on 8949 this year and no mailing required. Not sure how online handles this.
