How to get force constants in internal coordinates in Gaussian?
You have to use freq=intmodes and use the command

formchk -3 xxxx.chk xxxx.fchk 


to convert the binary checkpoint file to a formatted checkpoint file, which consists of text. The -3 requests the version 3 of formatted checkpoint files, which support all features of Gaussian 09 and 16.

Source: Ruixing Wang's answer on Researchgate, where I asked the same question.