If you want to submit your R package to CRAN, you need to ensure that all R checks will be executed successfully without a warning for the current and the devel version of R.
And not only that, as a package maintainer, you also need to ensure that your submitted packages will still work with upcoming R versions. If you are using native code in your package which needs compilation (e.g. C/C++), the whole process is even getting more complex, since you need to ensure that the compilation of the code works on different systems (debian, fedora, windows (32 and 64 bit), solaris (sic), and osx) and with different compilers (gcc, clang).
But, for the moment let’s focus on setting up an environment to check your package against the latest R-Devel version to be able to reproduce and resolve issues.