Error in setting up referencing with Natbib plain style: \providecommand\NAT@force@numbers{}\NAT@force@numbers

Question

I've set up my referencing and it's worked perfectly before in this same format (Harvard style) for all citations and the bibliography, but when I added a new citation it threw my pdf ouput completely out of whack.

It works when I do this:

\documentclass[a4paper,12pt]{article}
\usepackage{natbib}
\bibliographystyle{plain}
\begin{document}
This is an essay about \cite{clinch2006environmental} and \cite{aubert2019environmental}.
\bibliography{EnvironmentalEssay}
\end{document}

I get the output:

I want it in the Harvard style, so I changed to:

\documentclass[a4paper,12pt]{article}
\usepackage[round]{natbib}
\bibliographystyle{plain}
\begin{document}
This is an essay about \citet{clinch2006environmental} and \citeyearpar{aubert2019environmental}.
\bibliography{EnvironmentalEssay}

\end{document}

But then I suddenly get:

With the error: \providecommand\NAT@force@numbers{}\NAT@force@numbers "Package natbib Error: Bibliography not compatible with author-year citations"

And warning: that my author is undefined for first citation and year undefined for the second citation. I find this very strange though, since my Bibtex file is complete with all the information.

*EDIT: here is my Bibtex entries for the two sources:

@Article{aubert2019environmental,
  author    = {Aubert, Diane and Chiroleu-Assouline, Mireille},
  journal   = {European Economic Review},
  title     = {Environmental tax reform and income distribution with imperfect heterogeneous labour markets},
  year      = {2019},
  pages     = {60--82},
  volume    = {116},
  abstract  = {Text here xxx.},
  publisher = {Elsevier},
}
@Article{clinch2006environmental,
  author    = {Clinch, J Peter and Dunne, Louise and Dresner, Simon},
  journal   = {Energy Policy},
  title     = {Environmental and wider implications of political impediments to environmental tax reform},
  year      = {2006},
  number    = {8},
  pages     = {960--970},
  volume    = {34},
  abstract  = {text here xxxx.},
  publisher = {Elsevier},
}

It worked perfectly before for all commands:

\citep{}
\citet{}
\citeyearpar{}
etc.

However, since I added a new citation in my document, its producing errors.

I've tried changing the options on natbib, but nothing has worked. The time it worked perfectly I only had \usepackage[round]{natbib}.

*EDIT: I have tried using options [authoryear,round] option, but still has same outcome. I have now also changed the bibliographystyle from {plain} to `plainnat, with no success.

Please advise.