You don't have any public SSH keys in your GitHub account. You can add a new public key, or try cloning this repository via HTTPS. code example

Example 1: add ssh key to github

### ADD SSH key to Github  ###

# Check if you have a key?
ssh-add -l

# If not? then follow steps to generate key
ssh-keygen -t rsa -b 4096 -C your__gmail

# Now for copy the key use xclip tool or just goto file and copy
sudo apt install xclip
xclip -sel clip < ~/.ssh/id_rsa.pub

# Now goto github.com and goto settings 
# select  SSH and GPG keys
# New SSH key
# Enter "your__choice__name" in title field...
# Paste your public key into the Key field
# Click Add SSH key

DONE!!!

Example 2: add ssh key to github

$ ssh-keygen -t rsa -b 4096 -C "[email protected]"

Example 3: add ssh to github

$ clip < ~/.ssh/id_ed25519.pub
# Copies the contents of the id_ed25519.pub file to your clipboard

Example 4: add ssh key to github

> Enter passphrase (empty for no passphrase): [Type a passphrase]
> Enter same passphrase again: [Type passphrase again]