New Sentinel_1_B Radiometric Terrain Flattening Problem

Hi There
In the pre-processing of Sentinel 1-b images that I have (from late 2017 and 2018) Radiometric Terrain Flattening gives me a black image at the end. I tried all types of DEMs but the problem stays. Any idea what can I do? And what issue I will face if I skip this step?

Does your area lie outside the SRTM coverage?

You only need Terrain Flattening if you want to quantitatively analyze the backscatter in areas of pronounced topography.
You can skip it by calibrating to Sigma0 instead and proceed directly with the Range Doppler Terrain Correction.