How to generate a good symmetric secret key I would like to generate a good secret key, that later will be used to encrypt data symmetrically.
I am not sure how to generate a good secret key.

I use Linux, I am not sure which sources of enthropy there are.
The idea now is the following:
I can access current timestamp and seed with it PRNG to get some pseudo random value R.
Feed this R into some KDF, for example HKDF and use the result as a symmetric key.
Are there are some loopholes in such scheme?
A "good" secret key should have the same probability as any other possible key to be chosen.

No matter what extractor you will be using, if your entropy source is limited to the current timestamp, this is very unlikely to be the case.

Unless you have access to a hardware random number generator, you should combine as many independent entropy sources as possible.

As to the actual entropy collection, answering your question is impossible without knowing what your environment is.