Do you or did you have a yearly/monthly subscription to Lightroom Classic or was the formerly installed version a one-time purchase ("perpetually licensed")?
If you had/have a subscription to Lightroom Classic, the last version that ran on Mac OS 10.13 was LR 9. Unfortunately, Adobe only makes LR 10 and 11 available for easy download. You can download LR 8 from here:
But if you want LR 9, you'll have to contact Adobe support, and you might have to try several times, since support often gives the wrong answer:
If you had a perpetually licensed version, you can install the last such version (6.14) following these instructions:
You'll need your serial number for LR 6.14, which you can get by logging into your Adobe.com account if you purchased directly from Adobe or registered your purchase with Adobe.com.