How to formulate equality predicate on POINT column in PostgreSQL?
Try as it might, operator type resolution will find no appropriate operator and give up with the error message you reported.


ERROR:  operator does not exist: point = unknown



Adding an explicit type cast (or the point() function achieving the same) to the right argument like you did in your added second attempt will only succeed in making the error message a bit more revealing:


ERROR:  operator does not exist: point = point



The subtle difference may serve as hint: the problem is not a missing cast (though being explicit about data types is never a bad idea). The problem is that there is, in fact, no equality operator = defined for the data type point. The manual:


  Note that the “same as” operator, ~=, represents the usual notion of
  equality for the point, box, polygon, and circle types. Some of these types
  also have an = operator, but = compares for equal areas only. The other scalar
  comparison operators (&lt;= and so on) likewise compare areas for these types.


For the data type point, there is no = at all. See for yourself (tested in Postgres 12):

SELECT *            -- no rows
FROM   pg_operator
WHERE  oprleft = 'point'::regtype
AND    oprname = '=';


The solution is to use the appropriate "same as" operator ~= instead. Works with or without explicit type cast, with or without enclosing parentheses in the string literal, and with the function notation you tested as well - and in this case with either type of string literal as well as with two separate parameters as there are multiple overloaded versions of the point() function:

SELECT a.* FROM indsolv.address a
WHERE a.location ~= point '(93.2321, 21.0321)';

... WHERE a.location ~= point '93.2321, 21.0321';
... WHERE a.location ~= '93.2321, 21.0321'::point;
... WHERE a.location ~= '(93.2321, 21.0321)'::point;
... WHERE a.location ~= cast('93.2321, 21.0321' AS point);
... WHERE a.location ~= cast('(93.2321, 21.0321)' AS point);

... WHERE a.location ~= '(93.2321, 21.0321)';
... WHERE a.location ~= '93.2321, 21.0321';

... WHERE a.location ~= point('93.2321, 21.0321)';
... WHERE a.location ~= point('(93.2321, 21.0321)');
... WHERE a.location ~= point('93.2321', '21.0321');


Related:


ERROR:could not identify an equality operator for type point
Creating custom “equality operator” for PostgreSQL type (point) for DISTINCT calls