Composite Primary Key in Table in Sql server

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database

SQL-Server-2012

I want to create Composite Primary Key in a table, but Ms Sql server gives me the error that you cant create composite primary keys. why? or any alternative.
I want to create One-to-One relationship between Table Subscription against Table Payment then then One to One relationship against Table Payment to PaymnetCash,PaymentWesternUnion,PaymentCreditCard.

Create Table TblSubscription
(
SubscriptionId int identity(1,1) Primary key,
UserId INT NOT NULL Foreign key References TblAppUser(UserId),
PackageId INT NOT NULL Foreign key References TblPackages(PackageID),
StartDate datetime,
FinishDate datetime,
IsActive bit,
IsPaid bit
);


Create Table TblPayment
(
PaymentId int identity(1,1) Primary Key,
SubscriptionId int Primary key,
PaymentAmount float,
PaymentDate datetime,
PaymentMethod varchar(50),
PaymentStatus bit
);