Cannot Drop Index and PK Constraints

I am trying to create a new column on a table, but realized that there is already an PK index created. I am unable to drop the PK, or even delete the table. I also am unable to delete the index.Please be more specific - what error messages are you receiving ? Is it because of a system error or a constraint error (like you are deleting a primary key which is a foreign key in another table(s)) ?