After installing Python 3.9.6 on Ubuntu 18.04, I could not run sudo apt

I didn't know that Python 3.6 had already been installed by some package before I installed Python 3.9.6 onto my system.

I followed the steps found at https://phoenixnap.com/kb/how-to-install-python-3-ubuntu#ftoc-heading-7

1. sudo apt update
   

2. sudo apt install build-essential zlib1g-dev libncurses5-dev libgdbm-dev libnss3-dev libssl-dev libreadline-dev libffi-dev wget
   

3. cd /tmp
   

4. wget https://www.python.org/ftp/python/3.9.6/Python-3.9.6.tgz
   

5. tar -xf Python-3.9.6.tgz
   

6. cd python-3.9.6
   

7. ./configure --enable-optimizations
   

8. (Overwrite Default Python Installation)

   "To install Python 3.9.6 over the top of your existing Python, enter the following":

   sudo make install
   

After these steps, I found out that I have 3 versions of python:

$ python -V
Python 3.6.9

$ python2 -V
Python 2.7.17

$ python3 -V
Python 3.9.6

I then ran:

sudo apt update

The terminal returned:

File "/usr/local/bin/apt", line 6
    print "apt"
              ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print("apt")?

I could run other commands like sudo apt-get ... etc.

Could someone please tell me what went wrong?

Edit: Thank you Joshua Johns, steeldriver & vanadium for your help. I did the followings to verify:

$ ls -l /usr/bin/python
/usr/bin/python -> /etc/alternatives/python
$ ls -l /etc/alternatives/python
/etc/alternatives/python -> /usr/bin/python3
$ ls -l /usr/bin/python3
/usr/bin/python3 -> python3.6
$ ls -l /usr/bin/python2
/usr/bin/python2 -> python2.7
$ which python3 /usr/local/bin/python3
$ ls -l /usr/local/bin/python3
/usr/local/bin/python3 -> python3.9

To add on @vanadium whenever you want to install latest python in your machine always check the release date for both python and Ubuntu.its better to upgrade Ubuntu to latest release without breaking everything.for instance python 3.9 was released on October 5th, 2020 while Ubuntu 18.04 was released on July 26, 2018.for now you have to wait for Ubuntu 22.04 which most probably will have python 3.9 by default. Hopefully you get the point now.

I use deadsnakes PPA to safely install the latest python on Ubuntu 18.04 (20.04 installs 3.8 but has 3.9 in its official repository) without overwriting the existing system python. See this gist for details: https://gist.github.com/plembo/6bc141a150cff0369574ce0b0a92f5e7 . Note that my workflow doesn't require the latest python system-wide, so I always add the "--user" flag when installing additional libraries with pip.