Reading coordinates in tiff images using PyQt

[hobbyist]

Hello, it shows me this:

Error:
UbuntuUser@ubuntu:~/Desktop/PyQT$ pip3 show opencv-python
WARNING: Package(s) not found: opencv-python

I re-install OpenCV with these commands:

sudo apt update
sudo apt install libopencv-dev python3-opencv

and I still get the message above...What is wrong?

update: I ran this command:

pip install opencv-python

and it seems opencv to be installed.
When I try to run the python_code.py file, I get this error:

Error:
QObject::moveToThread: Current thread (0x21735e0) is not the object's thread (0x28819c0).
Cannot move to target thread (0x21735e0)

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "/home/mojito/.local/lib/python3.8/site-packages/cv2/qt/plugins" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: xcb, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, webgl.

Aborted (core dumped)

I use a small .tiff (about 8 MBytes)

I installed again opencv:

pip install opencv-python-headless

the above error has gone, but when I continue to see this error:

cv2.error: OpenCV(4.4.0) /tmp/pip-req-build-h2062vqd/opencv/modules/imgproc/src/color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cvtColor'

I use Ubuntu 20.04.1 and opencv version: Version: 4.4.0.46