Issue: When trying to install a package, you get the error message
No space left on device
Root cause: Too little space available on your device's flash. (happens easily on devices with only 4MB flash)
By removing packages you installed after flashing OpenWrt onto your Router. You cannot remove packages on the SquashFS partition, which is included in the image you flashed.
This happens easily with recent firmware on 4MB Flash devices.
You can press-fit an OpenWrt image into this small flash by building your own image, with only the packages you need, tailored for your usecase.
You can build your own image and remove packages that are not needed for your usecase