A problem with bibliography in Aberdeen OverLeaf dissertation template document

Question

I use this template. See this question of mine for context.

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The document is quite complex, so I'm not sure what to share here in terms of code.

I have this:

\begin{thebibliography}{99}
\bibitem{Bergelson} Bergelson, V., Best, A., and Iosevich (2020). A, Sums of Powers in Large Finite Fields: A Mix of Methods \textit{The American Mathematical Monthly} (2021), 128:8, pp. 701-718.
\end{thebibliography}

However, when I load it, TeXnicCentre says there is an error. When I click of the error, the following shows up:

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The problem, of course, seems to be in how I have entered the reference.