You have not defined R column type. If you want it to do want I guess you mean then you can use wr (with a relatively recent version of the array package) But this is forcing the column widths to values that are inappropriate for the column contents, and forcing the total table width to not fit the page. Just using a natural widths makes the table easier to read, or better remove the vertical lines and use booktabs package.

\documentclass{article}
\usepackage{array,booktabs}
\begin{document}
\begin{table}[ht]
\centering
\begin{tabular}{wl{0.015\textwidth}|wr{0.37\textwidth}|wr{0.12\textwidth}|wr{0.08\textwidth}|wr{0.02\textwidth}|wr{0.35\textwidth}|}
\hline
& Sepal.Length & Sepal.Width & Petal.Length & Petal.Width & Species \\
\hline
1 & 5 & 4 & 1 & 0 & setosa \\
2 & 5 & 3 & 1 & 0 & setosa \\
3 & 5 & 3 & 1 & 0 & setosa \\
4 & 5 & 3 & 2 & 0 & setosa \\
5 & 5 & 4 & 1 & 0 & setosa \\
6 & 5 & 4 & 2 & 0 & setosa \\
\hline
\end{tabular}
\end{table}
\begin{table}[ht]
\centering
\begin{tabular}{r|r|r|r|r|l|}
\hline
& Sepal.Length & Sepal.Width & Petal.Length & Petal.Width & Species \\
\hline
1 & 5 & 4 & 1 & 0 & setosa \\
2 & 5 & 3 & 1 & 0 & setosa \\
3 & 5 & 3 & 1 & 0 & setosa \\
4 & 5 & 3 & 2 & 0 & setosa \\
5 & 5 & 4 & 1 & 0 & setosa \\
6 & 5 & 4 & 2 & 0 & setosa \\
\hline
\end{tabular}
\end{table}
\begin{table}[ht]
\centering
\begin{tabular}{cccccl}
\toprule
& Sepal.Length & Sepal.Width & Petal.Length & Petal.Width & Species \\
\midrule
1 & 5 & 4 & 1 & 0 & setosa \\
2 & 5 & 3 & 1 & 0 & setosa \\
3 & 5 & 3 & 1 & 0 & setosa \\
4 & 5 & 3 & 2 & 0 & setosa \\
5 & 5 & 4 & 1 & 0 & setosa \\
6 & 5 & 4 & 2 & 0 & setosa \\
\bottomrule
\end{tabular}
\end{table}
\end{document}
RandCare not standard column types. – Joseph Wright Apr 29 '20 at 08:551\textwidth), please see the posting How to force a table into page width? The two leading candidates are thetabularxandtabular*environments. – Mico Apr 29 '20 at 09:07