More than one user may be reading and editing the same data at the same time. To be able to work with your data, the application must be able to assume that the schema is fixed and is not changing at any time that you are working with its contents.
Check that you are exclusively using and editing your dataset. The most common cause of a schema lock is displaying a dataset in ArcMap while you attempt to work with the data in another application. Learn more about rules for working with schema locks.