RStudio: Go from disabled citation insert to enabled citation insert.

Can you tell us what you are doing? As far as I can see there is no citation insert in basic RStudio. Are you working in a Rmd document?
Do you have something like

bibliography: Roman.bib
csl: apa.csl

in the YAML?

If so, try knitting the document. For some reason the Rmd file sometimes seems to fail the recognize the bibliography until after an initial knit after the document is loaded.

Otherwise can you supply us with a reproducible example (reprex)