I hate to be the bearer of bad news, but I don't think that really is a Huawei Mate 9.
According to Storage Truth (which was written by one of our moderators, and which I've always found reliable on every devices I've tested it on) that phone has about 4GB of storage: 1.5GB /system, 1.7 GB /data, and add a bit for radio, cache etc and my guess is 4GB total. A Mate 9 should have 64GB. Also it says it's running Android 5.1, but the Mate 9 launched with Android 7.0 and no Mate 9 has ever run 5.1. And that "device" field: gxq6580 is not the model id of the Mate 9, but it's found in a range of different "clone" phones based on the MediaTek MT6580 (a low-power budget SoC) but pretending to be all sorts of different phones.
So I'm afraid that's why you are running out of space: that phone has virtually none. And the second screenshot is showing 62GB free because it's subtracted your actual usage from a fictitious 64GB: it has been programmed to lie about how much storage the phone has.
If the person who sold you this said it was a real Mate 9 you definitely want your money back. Actually I'd say you want your money back anyway: with that little storage (1.7 GB) there is no way you will be able to use it as a smartphone.
Sorry, but I really do think that is the situation. It also explains why your phone is simultaneously telling you that it's got no space and that it's got more space than a Mate 9 should ever have: the phone is pretending to be a (much) higher-spec device than it really is.