Add generated column to an existing table Postgres

I am trying to add a generated column to an existing table with this script.

alter table Asset_Store add column

md5_hash VARCHAR(100) GENERATED ALWAYS AS 

(CAST(UPPER(    
        case
             when OR_ID is not null then MD5(cast(OR_ID as varchar(100)))
             when Asset_ID is not null then MD5(Asset_ID)
             else null
        end 
) as VARCHAR(100)))

STORED

;

but I am getting an error:

SQL Error [42601]: ERROR: syntax error at or near "("
 Position: 88
 ERROR: syntax error at or near "("
 Position: 88
 ERROR: syntax error at or near "("
 Position: 88

What is the issue? I don't get it.

In the schema of my Asset_Store table the column
OR_ID is int and Asset_ID is varchar(100).

I guess it expects a slightly different syntax... but what is the right syntax?