Package natbib Error: Bibliography not compatible with author-year citations

When changing the bibliography style, sometimes natbib is upset because it can't interpret the data correctly.

In any case, after changing the argument to \bibliographystyle a run of LaTeX and one of BibTeX are necessary to get back in sync. Removing the .bbl and .aux files before those run is recommended, in order to avoid spurious error messages that might corrupt the .aux file currently being generated.

I had the same error trying to use spmpsci.bst in conjunction with natbib. While natbib's default citation style is author-year, spmpsci.bst is numeric, so this is a strange error to get. I eventually changed the following:

\usepackage[square,sort,comma]{natbib}

to

\usepackage[square,sort,comma,numbers]{natbib}

Problem went away.

Another way this error can arise is if one of your BibTeX entries has no year.

To find out which entry it is, look earlier in the output for a message like Warning--empty year in ABC2019. So check ABC2019.