Cannot alter column 'X' because it is 'timestamp'.

How to Change DataType of TimeStamp Column.

Datatype of Timestamp column cannot be deleted. Hence it is advised to drop the column and re-create it

For example,

ALTER TABLE [dbo].[PerformanceMaster] DROP COLUMN PerformanceDate
ALTER TABLE [dbo].[PerformanceMaster] ADD PerformanceDate datetime2