problem comand EXECUTE IMMEDIATE or privileges on tables

  something  curious is that is I execute the comand in Sql plus or TOad  and the comand executes  excellent

 i put think question in Pl_SQL too so I don´t I the problem is comand that I think is ok or  the problem really is about privileges

In a table on database ,   I have the comments of the columns of data base , so I like to pass this comments  to the database comments so  I use the EXECUTE IMMEDIATE  comand like this :

where tabacampo  is string that for example coudl be :  compania.id_compania  and

      comentario   is the comments thats I like to add  on table... the problem is I think the command is ok