It's hard to know for sure, since you are not providing a REPRoducible EXample (reprex), but I suspect that your problem is with the YAML header for your .Rmd file instead of your code for the steamgraph.
Take a look at this example.
---
title: "Untitled"
output:
flexdashboard::flex_dashboard:
orientation: columns
vertical_layout: fill
---
```{r setup, include=FALSE}
library(flexdashboard)
library(dplyr)
library(babynames)
library(streamgraph)
```
Column {data-width=650}
-----------------------------------------------------------------------
### Chart A
```{r}
babynames %>%
filter(grepl("^Kr", name)) %>%
group_by(year, name) %>%
tally(wt=n) %>%
streamgraph("name", "n", "year")
```
Column {data-width=350}
-----------------------------------------------------------------------
### Chart B
```{r}
babynames %>%
filter(grepl("^I", name)) %>%
group_by(year, name) %>%
tally(wt=n) %>%
streamgraph("name", "n", "year", offset="zero", interpolate="linear") %>%
sg_legend(show=TRUE, label="I- names: ")
```
### Chart C
```{r}
plot(iris)
```
