Plot command followed by lines command in rpres

Question

I have a .rpres file "Introduction to R.rpres". I use the preview mode in Rstudio to generate a presentation with several slides.

The following code enclosed by 3 ticks at the beginning and end

```{r,out.width='\\textwidth', fig.height = 8, fig.align='center'}
plot(NINDS_minimal_t_PA$NIHSSB, NINDS_minimal_t_PA$Dead)
abline(lmdeath_tpa)

Produces a plot on the slide

The following code

```{r,out.width='\\textwidth', fig.height = 8, fig.align='center'}
plot(NINDS_minimal_t_PA$NIHSSB, NINDS_minimal_t_PA$Dead)
lines(NIHSStest, Prop_dead)

produces message shown below as a slide. But no plot is generated

<img src="Introduction R-figure/unnamed-chunk-37-1.png" title="plot of chunk unnamed chunk-37" alt="plot of chunk unnamed-chunk-37" width="\textwidth" style="display: block;margin:auto;" ">/

How would I be able to generate a plot without the message?