I've experienced that Homebrew (brew) sometimes can end up with broken taps, causing software install to fail. As usual, the error messages could be totally misleading. This is what happened to me as I was trying to upgrade a previously well working Python3 on my machine.
I had run
$ brew upgrade and I got the following error message:
==> Upgrading python3 Error: No available formula for tcl-tk
So you'd immediately start looking for tcl-tk, and you'd be wrong. The real issue is that Homebrew taps is broken. Here is how you fix that, type this:
$ brew tap --repair
When the command is done, resume your install and everything works fine.