I am getting the same error. I will follow the steps and report back if it works or not.
Error in readRDS(pfile) :
cannot read workspace version 3 written by R 3.6.1; need R 3.5.0 or newer
Calls: source ... library -> find.package -> lapply -> FUN -> readRDS
BTW This is really a misleading error message.
EDIT: I have re-installed R 3.3.3 for windows, because I am running SQL Server 2017. It seems working, but I got conflicting dependency errors....
EDIT 2: After I downloaded dependent packages for version 3.3.3 from https://cran.r-project.org/bin/windows/contrib/3.3/ and installed them to SQL Server 2017, it finally worked!