Your Logon does not have CREATE TABLE permission; you might not be able to make certain edits that require this permission.

You are creating or modifying a table in a database in which your logon ID does not have CREATE TABLE permissions and is not a member of the DBO role.  Check with the database administrator to make sure that you have the necessary permissions to create tables in the database.

Remember, a seemingly innocuous modification to an existing table can require CREATE TABLE permission.  For example, if you add a column to a table, the Visual Database Tools instruct SQL Server to delete the table, re-create it, and re-insert the rows it had contained.