I have a database in SQL Server 2008. I am unable to set Identity specification for a column CartId as shown in figure

I deleted the column and tried creating a new column and set isIdentity property but it’s not working, also I deleted the table and tried creating a new table (not using query) but not able to set isIdentity property
How to fix this issue??
Data Type of column must be set to
INTEGERor other numeric type.