конвертировать в varbinary
isnull( HASHBYTES('SHA2_512', @Password),convert (varbinary,'') )
Попробуйте это
alter procedure [dbo].[Update_Users]
(
@Id int ,
@StaffName nvarchar(50) ,
@Email nvarchar(100) ,
@UserName varchar(40),
@Password varchar(50),
@Phoneno nvarchar(50) ,
@Admin char(3)
)
as
begin
Update Users
Set StaffName=ISnull(StaffName,@StaffName),
Email=ISnull(Email,@Email),
UserName=Isnull(UserName,@UserName),
PasswordHash = isnull( HASHBYTES('SHA2_512', @Password),convert (varbinary,'') ),
Phoneno=Isnull(Phoneno,@Phoneno),
[IsAdmin]=Isnull(IsAdmin,@Admin)
where Id=@Id
end