Nested IF NOT EXISTS in SQL server

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SQL

server

IF NOT EXISTS(SELECT * FROM sys.objects WHERE type = 'PK' AND  parent_object_id = OBJECT_ID ('IN_LOOKUP'))
BEGIN 

	IF EXISTS (SELECT 1 FROM IN_LOOKUP WHERE LID IS NULL )	
		DELETE FROM IN_LOOKUP WHERE LID IS NULL
	
	print cast(@@ROWCOUNT  as varchar)+' '+  'Null records deleted..'

	IF EXISTS(SELECT 1 FROM (SELECT LID,COUNT(1) DUP FROM IN_LOOKUP	GROUP BY LID HAVING COUNT(1)>1) B)
	BEGIN
		WITH  DeleteDup 
		 AS
		(
		SELECT ROW_NUMBER() over(Partition by lid order by LID ) DUP ,* FROM IN_LOOKUP (NOLOCK)
		)

		DELETE from DeleteDup WHERE DUP>1
	END

	print cast(@@ROWCOUNT  as varchar)+' '+  'Dupilcate records deleted..'
	
	IF EXISTS (SELECT COL.IS_NULLABLE FROM INFORMATION_SCHEMA.COLUMNS COL WHERE COL.TABLE_NAME = 'IN_LOOKUP' and  COL.COLUMN_NAME='LID' AND COL.IS_NULLABLE ='YES')
			ALTER TABLE IN_LOOKUP ALTER COLUMN LID INT NOT NULL		
	
	IF NOT EXISTS(SELECT 1 FROM sys.objects	WHERE type = 'PK' AND  parent_object_id = OBJECT_ID ('IN_LOOKUP'))
			 ALTER TABLE IN_LOOKUP ADD CONSTRAINT PK_IN_LOOKUP PRIMARY KEY (LID)
		 
	PRINT 'Primary Key added'
END

Getting Errors Like:

Msg 8111, Level 16, State 1, Line 28
Cannot define PRIMARY KEY constraint on nullable column in table 'IN_LOOKUP'.
Msg 1750, Level 16, State 0, Line 28
Could not create constraint or index. See previous errors.

What I have tried:

I tried to solve but the cursor directly coming to IF NOT Exist statement in side the IF block.
Can any one help me out of this.