First Order filter dont work in 20kHz

Hi, my name is Andre and i'm working with Eval-ADAU1701Miniz. I'm using a first-order filter to make a project, but in the frequencies of 10kHz up to 20kHz the filter dont work. If i put a frequency above of the 20kHz, by example a 24kHz, the signal became pulsative in low frequency. I attached an image of the project, the signal and gain. Thanks for help.