Can I know how to resolve the error "No public key found for key"

[Makoto Yamashita <Makoto.Yamashita@c.titech.ac.jp>]

Dear whom it may concern,

Thank you very much for reading my email.
I am a maintainer of the SDPA package and I am trying to
upload a modified file, but I received the following mail.
====
  Unable to verify file sdpa_7.3.16+dfsg-1_amd64.changes. No public
key found for key ....
  Please try to fix it and re-upload. Thanks,
====

I am trying to search for information related to this message.
However, I cannot find any good information to resolve this issue.
If there is some info, could you please share it with me?
(I made my PGP key almost 10 years ago.
  This may affect the above error, but I do not know well.)

It was suggested that I ask this on this mailing list,
so I am writing this mail.

Thank you very much in advance.

Best regards,
Makoto Yamashita