It means there is something not right with the tcltk package. You need to look into this on your environment.
Small hints: this package is used to create some user interface usually, and is usually used for interactive session. When you render a Rmd document, it will be in a non-interactive session, and you can't execute a function into a chunk that will need to open a window or something else.
Without any example of what you tried, we won't be able to help much more.
Please share an example of what you tried.