Fatal error in launcher: Unable to create process using "" (English)

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I could not run some pip commands...

・Caution

I am not professional, so what I will tell you may be incorrect. I recommend you to check what you read are really correct comparing with other sources (and if you find any mistakes, please tell me about it).

・Background

I was installing Python and pip to my desktop PC. First I installed Python3. After a short time, I realized that Python2 were also needed in my use, so I installed it.

Unfortunately, after that, some articles on the Internet says that if not install Python2 before installation of Python3, I cannot control Python version. (Perhaps, there are other ways.)

Therefore, I uninstalled Python2 and Python3 through Control Panel, and reinstalled in the order of Python2 and Python3. In this way, I succesed installation of them.

However, when I tried to run pip command via command prompt to make sure that installation was succeed, some pip command did not work and the following error message was shown.
　　Fatal error in launcher: Unable to create process using "" ""　　
This message was displayed when I use the command like
　　pip "something"
If pip command work correctly, it is predicted that pip3 (pip of Python3) will run, but it didn't, though commands like
　　py -m pip "something"
worked...

・Environment

Windows10 Pro
Python3.7.1
python2.7.15

・What I Did.

First, I made sure that the path was set correctly. If you have never checked the path, you should do that referencing other article.

Second, I checked the path in Python and I confirmed that "bad path" did not exited. (*1)
I heard there is a path in Python, but I do not know well about it.

Moreover, I tried to update pip using the command below.
        py -m pip install -U pip　
To my disappointment, it told me that the version of pip was latest.

Finally, I did reinstall Python further again and I did it  strictly. It solved the problem...
I uninstalled both Pythons and deleted all remaining files exited in the directory where Python were. In addition, I also deleted data related to pip which were in C:\Users\"UserName"\AppData\Local. After that, I install python2 followed by python3.

*1

I do not think that checking it has meaning, but I write how did I check it just in case.
Via command prompt, I started Python and typed the next codes.

        C:\******> python
       >>> import sys
       >>> sys.path

After that, some directories were shown and I checked them.