Thanks for replying to my message posted yesterday. But what is exactly nmake? Is it a file that is missing? If yes, where can I find this file? Thanks for your help once again!

You need to read Fluent's instructions for setting up and compiling (or interpreting) udfs. You will find their detailed instructions with the documentation that comes with Fluent. I can't help you with all the details here.

As a side note, I would generally recommend looking for another way to set-up and solve a problem, rather than a udf, even if it is a compromise. From my experience, you would need to allocate about a week to design, set-up, and test even a simple udf. And, if you have no idea on how to even get started, you can double or triple that estimate.