I believe you need to use the library(DomoR) 

and then the command would be 

> DomoR::create(...)

@user02371 It sounds like you may not have loaded the library so R doesn't know where to look for it. You could either load the library by adding

library(DomoR)

somewhere before the call or do as @ST_-Superman-_ suggested and run it as follows.

DomoR::create(X, "Sales Forecast : XYZ", " Description")

If you have tried both of those things, please upload the entire script (remove your access token first) and we can see what else might be happening.

You shouldn't need to do both of these things. Calling "library" at the beginning of your script adds the sepcified libray to the search path so whenever you call a function R will search the DomoR library for the create function. If you call DomoR::create then R will automatically get the function from the DomoR library without searching.