GPG : "No secret key found" error

[Robert J. Hansen]

> I am writing this email to you in plain text... I am surprised how is it 
> coming to as HTML.

As I don't use GMail, I can't help you.  You'll need to ask Google. 
Your message comes through as having both plaintext and HTML parts. 
This, for instance, is part of the source of your email:

Content-Type: text/html; charset="UTF-8"

Content-Transfer-Encoding: quoted-printable



<div dir=3D"auto">I am writing this email to you in plain text... I am surp=

rised how is it coming to as HTML.<div dir=3D"auto"><br></div><div dir=3D"a=

uto">Any idea?</div><div dir=3D"auto"><br></div><div dir=3D"auto">Any speci=

al things I need to check before sending the email?<br><br><div data-smartm=

ail=3D"gmail_signature" dir=3D"auto">-Regards<br>Abhisht Sharma<br>+61 4204=

10228</div></div></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" clas=

s=3D"gmail_attr">On Thu, 10 Jun 2021, 02:58 Robert J. Hansen, <<a href=

=3D"mailto:rjh at sixdemonbag.org">rjh at sixdemonbag.org</a>> wrote:<br></div=

 ><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 
.8ex;border-left:1=

px #ccc solid;padding-left:1ex">I'm not going to respond to this until =

you re-send it as plain text <br>

without HTML.=C2=A0 The very first thing I wrote in my last email was that =

<br>

this mailing list strongly prefers plain text without HTML.<br>

<br>

We're willing to help you, but you need to follow the rules.<br>

</blockquote></div>