:db.error/not-an-entity Unable to resolve entity: :db/index

When working with Datomic Ions, I wanted to use :db/index on an attribute but got this error:

:db.error/not-an-entity Unable to resolve entity: :db/index

Using the magic of Google I came across this tidbit:

robert.mather.rmm00:11:22

On Datomic Ions Solo, I'm getting an error Unable to resolve entity: :db/index when trying to transact schema

robert.mather.rmm00:11:30

oh, is :db/index not supported on Datomic Cloud?

robert.mather.rmm00:11:12

I guess fulltext indexes aren't either...

henrik05:11:22

Alas, no. And be careful with large strings as well.

Summary

It appears that :db/index and :db/fulltext are not support on Datomic Cloud.  The only documentation for this feature is in a slack log from November 2018.

https://docs.datomic.com/cloud/schema/schema-reference.html
https://docs.datomic.com/cloud/query/raw-index-access.html