Lack of mathematical work during PhD and coping up with the requirements of postdoc positions I am a recently graduated 31 years old PhD in a STEM field. I have been applying for postdoc positions for the past few months, and have received rejections from almost all of them, with few yet to respond. With the academic year coming to an end, I don't have any hope of getting a position and I will continue as a postdoc or Research assistant in my PhD group.

However, one common reason mentioned in all the responses to my applications was that I don't possess the required mathematical and programming skills to work on the research field. My PhD research involved using Fluent CFD software to model has flow in a nozzle. I did not develop any code for that. I used python programming to develop different nozzle geometries and used it to study the flow.

I did not take any Finite volume method or Finite element method course or fluid dynamics course in my undergraduate or graduate school as my supervisor told me to self learn the concepts. I have published 4 papers in decent journals.

However, all the postdoc positions I apply to require code development for CFD and having academic background with the numerical methods. And since I don't have any experience with code development, I don't know if I will be able to do the job considering my lack of formal training.

Is this dilemma expected after PhD? What should I do to improve my chances in getting a position?
If you have the knowledge, but not the "qualification" in those areas, make sure that your letters of recommendation say that. If you don't have the knowledge, it might be a bit easier to improve Python skills than math skills. 

Alternatively, you might be able to get someone to tutor  you in the math area and make sure your advisor and other recommenders know to mention it in the future. 

A third option is to focus your statement of purpose and other application materials on your knowledge of these things, rather than just listing courses.