Primary filegroup is full

I am trying to create a primary key (clustered across 6 columns on a 5 million row table)

When I execute my Create Index statement:

ALTER TABLE dbo.Fact_Prescriptions ADD CONSTRAINT
PK_Fact_Prescriptions PRIMARY KEY CLUSTERED 
(
    VarcharCol_1,
    VarcharCol_2,
    VarcharCol_3,
    VarcharCol_4,
    VarcharCol_5,
    VarcharCol_5,
    DateTimeCol_1,
    DateTimeCol_2
) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

I get the following error

Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.
The statement has been terminated.
Msg 1105, Level 17, State 2, Line 1
Could not allocate space for object 'dbo.SORT temporary run storage:  
422220863176704' in database 'XXXXXXXXX' because the 'PRIMARY' filegroup 
is full. Create disk space by deleting unneeded files, dropping objects 
in the filegroup, adding additional files to the filegroup, or setting 
autogrowth on for existing files in the filegroup.

How do I correct this?