Dear Karina.

You can access to the Cambridge database for free on the CCDC page.

https://www.ccdc.cam.ac.uk/structures/?

No, I didn't get the cif of dimethyl phosphate instead of this I get the cif file of ammounium dimethyl phosphate. Is there any way to convert this cif into desired one

First, dymethyl phophate is a liquid, getting a CIF for the solid form of this material alone will be difficult, if not impossible. In the literature there are no reports of attempts to study its structure in its solid form.

In the CSD database there are only several reports of metal salts or complexes containing this compound, in its acid form or in its basic form. The question is why you need that CIF, based on that several things can be done. If what you are looking for are the coordinates of this compound in its acid form for some calculations. The CIF file could be edited with a program like Mercury (https://www.ccdc.cam.ac.uk/solutions/software/mercury/) or Vesta (https://jp-minerals.org/vesta/en/) to remove the other components and generate the necessary file. The CSD entry CISKIY could be used for this purpose.

Many institutes like, IACS, Kolkata, Indian Institute of Science, Bangalore pay to Cambridge crystallographic Data Base and they have access to all published CIFs. If you know somebody there, you can request him or her. Else you try to locate the author, who published these CIF files and request him/her.