Sorry for asking the obvious: Are the SDA, SCL, GND and +5V wires are also properly connected?
This is most probably a network or software repository issue. These errors are not detected by version 3.3.1 of the plugin. This has the effect that the plugin only seems to have installed successfully but can’t work correctly. So I strongly recommend to use version 3.3.5. If the installation process aborts most often you just have to try to install the plugin repeatedly.
To get the display working you might have to do the following:
sudo curl -L https://github.com/waveshare/7inch-DSI-LCD-C/raw/main/32/5.10.92/pi4/Driver_package/WS_7inchDSI1024x600_Screen.ko -o /lib/modules/$(uname -r)/WS_7inchDSI1024x600_Screen.ko
sudo curl -L https://github.com/waveshare/7inch-DSI-LCD-C/raw/main/32/5.10.92/pi4/Driver_package/WS_7inchDSI1024x600_Touch.ko -o /lib/modules/$(uname -r)/WS_7inchDSI1024x600_Touch.ko
sudo curl -L https://github.com/waveshare/7inch-DSI-LCD-C/raw/main/32/5.10.92/pi4/Driver_package/WS_7inchDSI1024x600_Screen.dtbo -o /boot/overlays/WS_7inchDSI1024x600_Screen.dtbo
sudo curl -L https://github.com/waveshare/7inch-DSI-LCD-C/raw/main/32/5.10.92/pi4/Driver_package/WS_7inchDSI1024x600_Touch.dtbo -o /boot/overlays/WS_7inchDSI1024x600_Touch.dtbo
cd /lib/modules/$(uname -r)
sudo depmod
sudo modprobe WS_7inchDSI1024x600_Touch
sudo modprobe WS_7inchDSI1024x600_Screen
Then modify “/boot/userconfig.txt”:
cat >> /boot/userconfig.txt <<EOL
ignore_lcd=1
dtoverlay=WS_7inchDSI1024x600_Screen
dtoverlay=WS_7inchDSI1024x600_Touch
EOL
Reboot.
Please note I have no experience with this display and just extracted all that from Waveshare’s install script. No clue if it will work…