I am not able to modify the structure of a table in a database.
The database is used for replication.
I am getting This error
As the error says the table is used for replication, meaning it's an article in a replication publisher. To be able to modify the table you should remove it from replication, then update the schema, afterwards add it again in the replication. After adding it again you will have to reinitialize the subscriptions to pick up the modified table schema.