$ sudo apt install python3-pip
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package python3-pip is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package 'python3-pip' has no installation candidate
The first procedure you followed is correct
sudo apt-get -y install python3-pip
But before installing try to update using command
sudo apt-get update
If first did not work then you can also do this using curl
curl "https://bootstrap.pypa.io/get-pip.py" -o "get-pip.py"
python3 get-pip.py --user
if above not work:
curl -sS https://bootstrap.pypa.io/get-pip.py | sudo python
Then to verify installation try
pip3 --help
For checking version :
pip3 --version