I just had the same issue and did some research.
It seems that MSTL is only available on the most recent version of statsmodels: version 0.14.0
If you install statsmodels using conda install -c conda-forge statsmodels,
you will get the statsmodels 0.13.2 version.
(Using a script editor, try searching for 'MSTL' through C:\Users{username}\Anaconda3\Lib\site-packages\statsmodels, or wherever statsmodels is installed on your machine, you will probably not find it)
You'll need to install the most recent version from the latest source on statsmodels's github repository:
www.statsmodels.org/dev/install.html
From the anaconda prompt:
git clone https://github.com/statsmodels/statsmodels.git
pip install git+https://github.com/statsmodels/statsmodels
You will need a C compiler and git installed
For git you can use: conda install -c anaconda git
Be careful as the installation of the newest version may interfere with your other installed python packages.
I would recommend that you use a conda virtual environment for this.