How should R know that plm is a function?
Did you define the function yourself or does it belong to a package?
The function is not familiar to me.
And with the information you provide I can not help you.
Try to make a very small program with your problem. Show where the plm function comes from.
In the ideal case you would use the reprex package to do this but in any case read the article whats-a-reproducible-example-reprex-and-how-do-i-do-one .
Hope this helps.