Welcome to Android Central! Do you have an SD card inserted, and if so, did you format it as Internal Storage? I'm assuming that's the case, because the specs show that the highest internal storage version of that phone is 64 GB:
https://www.gsmarena.com/oppo_a3s-9269.php
Formatting an SD card as Internal Storage can be glitchy, depending on how well the manufacturer implemented the feature. I recall that Moto phones would still favor saving or installing apps to the actual Internal Storage instead of the SD card, thus leading to a low storage warning even though there was plenty of space on the SD.