Zelig has recently been updated and does not work in all cases. If you've installed the new version by mistake, follow the instructions below to go back to the old version 5.0.12:
If you already have Zelig installed, you can check its version using the packageVersion() function. Make sure it says version 5.0.12.
packageVersion("Zelig")
[1] '5.0.12'
If it's not 5.0.12 then proceed with the next steps to install the correct version.
First remove the package
remove.packages("Zelig")
Close RStudio and open it again. *** DO NOT SKIP THIS STEP ***
Install the package
install.packages("https://cran.r-project.org/src/contrib/Archive/Zelig/Zelig_5.0-12.tar.gz", repos=NULL, type="source")