If you are unable to send an email using any app, from Gmail, you need to allow it from your Google account.
Follow the below steps to allow Apps to send email.
Click on you account photo no top right corner in your browser, on google home page. A window will apper, now click on “Manage your Google Account”
After it click on “Scurity” tab.
Go to “Less secure app access” section
Turn on “Less scure app access”
Now, You will be able to send emails using any app.