The app you are looking for may have been removed from playstore or it is hidden from you because your android device is unsupported. An example i s that you cannot run android 4 apps on android 9 and neither can you run android 9 or 8 apps on android 6 and below. New versions of android use new sets of programiing codes. Just update your system or downgrade it depending on the app you need. I wouldnt recommend a downgrade though. It is not worth it.