Bioconda as a channel depends on Conda Forge and so specifying bioconda as the only channel is incorrect. Instead, a proper specification for using the bioconda channel is

name: freebayes
channels:
  - conda-forge
  - bioconda
  - defaults
dependencies:
 - freebayes=1.3.6

See the channel configuration in the Usage section.

TLDR

conda create -n beast -c conda-forge -c bioconda python=3.7 beast2

Longer version

By telling conda that it can visit conda-forge, it has some extra options to resolve the dependency conflicts. However, that doesn't really explain why this happens:

$ conda create -n beast python=3.7 libgcc-ng=7.3 libstdcxx-ng=7.3
$ activate beast
$ conda install -c bioconda beast2
Package libstdcxx-ng conflicts for:
python=3.7 -> libstdcxx-ng[version='>=7.2.0|>=7.3.0']
beast2 -> beagle-lib -> libstdcxx-ng[version='>=7.3.0|>=7.5.0']

It isn't clear why these dependencies aren't satisfied, since we already installed libstdcxx-ng version 7.3.

Alternative

If you instead use mamba to make the environment, you get slightly more helpful feedback:

$ conda install mamba
$ mamba create -n beast-mamba python=3.7 beast2
...
Problem: nothing provides requested beast2
...
$ mamba create -n beast-mamba -c bioconda python=3.7 beast2
...
Encountered problems while solving.
Problem: nothing provides openjdk 8.0* zulu8* needed by beast2-2.4.5-0
...
$ mamba create -n beast-mamba -c bioconda -c conda-forge python=3.7 beast2
...
Success

libgcc-ng, libstdcxx-ng do exist in conda-forge. But they're for Linux only, not for macOS, Windows.

• conda-forge/libgcc-ng
• conda-forge/libstdcxx-ng

These two packages include libraries for gcc. For macOS, install gcc with homebrew. They are contained by the gcc package already.

brew install gcc