I am not able to use the option for analysing a Reliability of a scale. The OK and Paste are inactive. I am using the one moth free trail.

You have to have at least two variables in the Items box to enable OK and Paste. Check out the help for the dialog box.

I have 22 variables in the Item box, this is not the problem.

I can't think of any other enabling requirement. To double check, this is Analyze > Scale > Reliability Analysis. Correct?

You could try running the syntax from the Syntax Editor window. It would look like this (variable names assumed to be var1 var2 ... var22. Change as needed.)

RELIABILITY

 /VARIABLES=var1 to var22

 /SCALE('ALL VARIABLES') ALL

 /MODEL=ALPHA.

Reliability analysis refers to the fact that a scale should consistently reflect the construct it is measuring. ... An aspect in which the researcher can use reliability analysis is when two observations under study that are equivalent to each other in terms of the construct being measured also have the equivalent outcome.