Invalid syntax, but not showing what it is!?!

[DeaD_EyE]

if condition:
    # code for this case
elif condition:
    # code for this case
else:
    # code if no case is True

In your case you have the colon at the wrong position.
A condition could be everything which can return a bool

A condition for your if/elif statement can be GPIO.input(7).
You can do also the equality check GPIO.input(7) == True which returns also a boolean.

If I guess, GPIO.input(7) should return an integer object. An integer object with the value 0 it False. If it has the value 1, it's True. 1 == 1.0 == True and 0 == 0.0 == False.

Your fixed code:

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BOARD)
GPIO.setup(7,GPIO.IN)

if GPIO.input(7)==True:
    print ("Secure")
elif GPIO.input(7)==False:
    print ("Intruder alert")

Or easier:

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BOARD)
GPIO.setup(7,GPIO.IN)

if GPIO.input(7):
    print ("Secure")
else:
    print ("Intruder alert")

We know that GPIO.input only returns 1 or 0.
There is no third case.

Little hint: If you want to check if an object is a boolean, just check it with bool(your_object). If the object has the special method __bool__, it returns True or False. Then you can use this object in an if/elif statement.