What type of diffractometer do you have? What is your detector? Is it CCD or image plate or a single GM detector. What is the vertical direction of the crystal if you do not have a four-circle diffractometer? You must answer all these question before someone can help you further. Is your thin film truely expitexial?

We have used 5 axis cradle diffractometer (Panalytical Empyrean) with PixCel 3D detector. The vertical direction of the crystal was (100).For reference I am attaching the RSM of the film.

I see at least one Bragg peak. The other peak on the right top looks very diffuse. Can you index these two peaks? You should explore more peaks by changing your detector positions (two-theta) and find more peaks. Once you have found several peaks and index them the lattice parameters can be calculated. Does you diffractometer have a soft ware? Normally the software comes with the instrument. The soft should do all these jobs rather routinely. Granted you do not have a standard X-ray single crystal, but only a thin film, which may or many not be truly epitexial, determination of lattice parameters should pose no problem because that requires only the diffractometer angles and the position of the peaks on the detector. The intensity values are of help but are not necessary for the lattice parameter determination. Determination of the structure is of course more difficult because that involves intensity values.

Try out this tutorial to get an Idea

http://www.doitpoms.ac.uk/tlplib/reciprocal_lattice/absences.php

Hi  Aziz ... are you able to find out the answer for the question.

I also have similar query.