There should be a space between uname and -r
We suggest to keep a backup copy of the makefile used to compile your kernel available in \verb|/lib/modules/5.14.0-rc2/build|.
A simple command as following should suffice.
\begin{codebash}
-cp /lib/modules/`uname-r`/build/Makefile linux-`uname -r`
+cp /lib/modules/`uname -r`/build/Makefile linux-`uname -r`
\end{codebash}
Here \sh|linux-`uname -r`| is the Linux kernel source you are attempting to build.