Google play errors on Android when downloading an app?

Question

Android

Hello guys!
Your help is urgently needed.
I tell you: Today, my mother, on her old smartphone, asked me to delete files and clean her phone (flash drive) on her phone.
I connected a smartphone to the computer and started cleaning only the USB flash drive, I didn’t go into the “phone” folder itself.
And, yes, I also turned on, when connected to a computer on a smartphone, something like a USB file browsing mode ... something like that, I don’t remember exactly, because otherwise the computer for some reason did not see the device.
Next, I deleted all sorts of videos, photos, and 4 of some kind of large homogeneous files (all about 50 and a few meters each. I don’t remember the name, but it was hardly something systemic, on a flash drive in general.
In general, I deleted files from my smartphone using a computer through a cord. And I decided to additionally put the application on the android through the play market "clean master-antivirus".
BUT when I tried to install, I got "error code 24". Previously, Clean master was already installed on android, but I deleted it.
I climbed on the Internet - they say that the easiest way to clean the cache of the play market is to clean it, it did not help. Also 925, in my opinion, the error got out once.
After that, the Internet disappeared - there is an "H +" icon at the top of the screen, but there is no Internet. I tried to distribute through Wi-Fi from my smartphone - it works. HZ can match and the traffic is over.
Then, this weird message popped up:

Image

Dangerous application - application center, I click "delete" nothing happens.
In short, some kind of garbage, what to do?
I can't root.