Greetings, lizette71.
It appears that your MacBook Air is displaying a white line across the display. Good job on running Apple Diagnostics, and also trying to restart in macOS recovery. We understand how concerning this can be, and we're glad to help with this.
Reset NVRAM or PRAM on your Mac – Apple Support -- Reset the NVRAM (nonvolatile random-access memory) of your MacBook Air to see if the line disappears.
About macOS Recovery - Apple Support -- If the line is still present, restart your Mac in macOS Recovery using the Command-R keys.
If the line is still present in macOS Recovery, please contact Apple Support: Get Support - After selecting this link, choose Mac notebooks and type "display" in the Search topics field. Select "Display issue" from the Results, then start your conversation with Apple.
Take care.