Greetings, anmolsek.
I see that you're unable to download songs to the Music application on your iOS device. You've also noticed that some of the album artwork isn't appearing, and I'm happy to help.
Get help with iCloud Music Library
First, check iTunes to see if the song is ineligible for upload. Additionally, if you added the song to your library from the Apple Music catalog, it might no longer be available on Apple Music.
Check your Internet connection
Open Safari on your iOS device or Mac, or a web browser on your PC, and go to any website to make sure that you have a strong Wi-Fi or Internet connection.
If you can't connect to a Wi-Fi network on your iOS device, you can try using cellular data:
- Go to Settings > [your name] > iTunes & App Store.
- Turn on Use Cellular Data.
You might have to pay a fee to download over a cellular network. Contact your wireless carrier for details, and learn about cellular data settings and usage.
And
- Find the album with the missing artwork, then right-click or Control-click it. To select more than one album, hold down the Command key on your keyboard and click additional albums.
- Choose Get Album Artwork. iTunes adds the album artwork.
- From the menu bar at the top of your computer screen or at the top of the iTunes window, choose File > Library > Update iCloud Music Library.
- After iCloud Music Library finishes updating, open the Music app on your iOS device.
- Find the songs that are missing artwork, and remove the downloads from your device.
- Tap to the right of the song to download the song again from iCloud Music Library.
Take care!