Unlike most of the structures, fossils can't be located via the /locate command or with the "feature" condition for advancements.
Linked issues
Comments


Petition to Re-Open as Nether Fossils can be located as of 20w11a
There's a new ticket for this: MC-174499
This issue has been closed as invalid.
You have posted a feature request or a suggestion. This site is for bug reports only.
For suggestions, please visit Minecraft Suggestions on Reddit.
Quick Links:
📓 Issue Guidelines -- 💬 Community Support -- 📧 Customer Support -- 📖 Game Wiki