You have specified the location in which to table is to be stored. You have explicitly specified a dbspace or the dbspace of the database has already been used. When you attempted to alter a table, you tried to change the dbspace in which the table is stored. This action is illegal. You can specify the dbspace only when you create the table.
Download any data in the existing table. Drop that table. Create a new table. Specify the dbspace that you want to use and upload the data into the new table.