Hello and welcome to the forum!
I've also tried running your code and it works for me:
library(ggplot2)
Month <- c(1,2,3)
Frequency <- c(100,200,300)
df <- data.frame(Month,Frequency)
ggplot(df, aes(x=Month, y=Frequency)) + geom_bar(stat="identity")

sessionInfo()
#> R version 4.0.4 (2021-02-15)
#> Platform: x86_64-apple-darwin17.0 (64-bit)
#>
#> Matrix products: default
#> BLAS: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRblas.dylib
#> LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib
#>
#> locale:
#> [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
#>
#> attached base packages:
#> [1] stats graphics grDevices utils datasets methods base
#>
#> other attached packages:
#> [1] ggplot2_3.3.2
#>
#> loaded via a namespace (and not attached):
#> [1] xml2_1.3.2 knitr_1.30 magrittr_2.0.1 tidyselect_1.1.0
#> [5] munsell_0.5.0 colorspace_2.0-0 R6_2.5.0 rlang_0.4.11
#> [9] httr_1.4.2 dplyr_1.0.2 stringr_1.4.0 highr_0.8
#> [13] tools_4.0.4 grid_4.0.4 gtable_0.3.0 xfun_0.19
#> [17] withr_2.3.0 htmltools_0.5.1.1 ellipsis_0.3.1 yaml_2.2.1
#> [21] digest_0.6.27 tibble_3.0.4 lifecycle_0.2.0 crayon_1.3.4
#> [25] farver_2.0.3 purrr_0.3.4 vctrs_0.3.5 curl_4.3
#> [29] mime_0.9 glue_1.4.2 evaluate_0.14 rmarkdown_2.5
#> [33] labeling_0.4.2 stringi_1.5.3 compiler_4.0.4 pillar_1.4.7
#> [37] generics_0.1.0 scales_1.1.1 pkgconfig_2.0.3
As you can see I am using R version 4.04. Have you tried downgrading?
Also, why is there an 'aes' object in the global environment? Maybe it doesn't matter, but I am just seeing one object when I run getAnywhere(aes):
> getAnywhere(aes)
A single object matching ‘aes’ was found
It was found in the following places
package:ggplot2
namespace:ggplot2
with value
function (x, y, ...)
{
exprs <- enquos(x = x, y = y, ..., .ignore_empty = "all")
aes <- new_aes(exprs, env = parent.frame())
rename_aes(aes)
}
<bytecode: 0x7fae89af68f8>
<environment: namespace:ggplot2>