iPhone won't backup - not enough storage

iCloud see -> Manage your iCloud storage

Manage your iCloud storage - Apple Support

See also ->  Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support

Mac see -> Free up storage space on your Mac

Free up storage space on your Mac - Apple Support

You can free up storage by deleting data on iCloud. Go to Settings > Apple ID > iCloud > Manage Storage, then select the type of data you want to remove. Then tap Delete Data. If you are disabling something like iCloud Photos, (by tapping Disable and Delete) you will have 30 days to download any data into files. You can do this at https://www.icloud.com/ . Hope this helps!