The file can succeed while the sitemap check reports a problem. Read the two results separately.
Open the sitemap URL on the storefront yourself. If it does not load, Shopify's sitemap needs attention. LLMs TXT will not write a replacement.
Two results
- Link count: the llms.txt entries
- Sitemap problems: the existing sitemap
- A warning does not roll back the file
- Fix the sitemap, then run the check again
Do not redirect sitemap.xml to llms.txt to clear the warning. That would hide the URL list search engines use.