We do not support Joomla Smart Search at this time. Joomla Smart Search works by indexing all the data rows into a singular searchable format. The problem with this is we have dozens of access checks, privacy checks, conditions, etc.. that can be applied to tabs, fields, and profiles. Those can't be applied to Joomla Smart Search and would become a serious problem for user data to be leaked like that. We brought this issue up on Github, but never really got an answer for how we can apply these checks.
We may eventually support it, but with a giant warning that privacy and access checks will not be applied. You can still use the Joomla 3 like search component on Joomla 4 though. It's just an optional install now from the JED.