library(shiny)
library(shinymeta)
library(dplyr)
ui <- fluidPage(
  selectInput("var", "Var", names(mtcars)),
  verbatimTextOutput("txt")
)
server <- function(input, output, session) {
  output$txt <- metaRender(renderPrint, {
    group_var <- sym(input$var)
    mtcars %>%
      group_by(!!group_var) %>%
      summarise(mean_mpg = mean(mpg))
  })
}
shinyApp(ui, server)

Error in enquo: object 'group_var' not found