This is the error message I receive when attempting the "user" function not the a!user function:
The user function does not have a domain prefix....
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the user function does have a domain - it's fn!
If you write it as fn!user() that message will disappear.
Some context: these are the available domains:
...and it's now been deemed best practice to prefix Appian constructs with their respective domain.
(just realized fn! doesn't appear in the above list, which seems to me an oversight! - I'll raise this with Appian Engineering)
...I also should have said that what you're seeing is not an "error" but a "recommendation". Appian has put investment into building implementation recommendations into the platform to help developers delivery quality applications. You can see these kinds of recommendations appearing in the Designer tools (like the Expression editor) and also in the 'Health Dashboard' tab of the 'Monitoring;' view within an Application.