If you've purchased a Waveshare screen and you're seeing lines on the display, it's usually due to one of two issues: Power or software configuration.
At the bottom of the product page for the specific Waveshare screen you've purchased, there should be a link to a setup guide. These guides include some changes you need to make to your Raspberry Pi to make the screen function properly.
One of the configuration/software changes increases the amount of current your Raspberry Pi USB ports can offer - giving the screen enough power to function properly. Without this, lines can appear and other functions may be unstable.
Similarly on the HDMI models of these screens, if you're trying to use them on a Windows device such as a PC or laptop you may encounter the same issue as many computer USB ports don't provide more than the required current.
Please use a dedicated quality power supply offering at least 2.5A for a Pi Zero/Pi3, and 3A for a Pi4 (we always recommend the official Raspberry Pi power supplies).
If you've checked the guide and configured your Raspberry Pi and are still experiencing issues - please get in touch with our support team.