Yes, and you very likely have one already without having done anything. Wix generates an llms.txt for your site and keeps it up to date, which puts it ahead of most platforms on this particular check.
Finding yours
Go to SEO & GEO in your site dashboard.
Scroll down to Tools and settings, then click through to llms.txt.
Read what is in it. It is generated from your own pages, so it is usually reasonable rather than perfect.
If the scan says you have no llms.txt and Wix says you do, open yourdomain.com/llms.txt yourself. That address is what we check, and it is the only opinion that matters here.
Editing it
You can edit the file through More Actions on that same screen. There is one consequence worth knowing before you do: once you edit it, Wix stops updating it automatically, so your changes are preserved and new pages are not added for you.
That is a fair trade if you have a Fix Pack file written from your own prices and services. It is a bad trade if you are going to add pages and forget. A guide pointing at pages that no longer exist is worse than none.
Your sitemap
Wix generates one automatically at yourdomain.com/sitemap.xml and there is nothing to create. If the finding mentions the sitemap, it is usually about the other half of that check: page titles that repeat, or canonical tags. Both are per-page settings in the SEO panel of the editor.
If your screens do not look like this, the wording has moved rather than the setting. Search your platform for the words in bold above. If you still cannot find it, send us the scan link and we will tell you where it is on your setup.