Having the following error when trying to add a new hypervisor to an existing cluster in VMware vCenter?
A general system error occurred: Time out waiting on vpxa to start
This is most likely happening with the ESXi version is greater than the ones supported by your currently running vCenter.
Have a look at the major, minor, revision and build number of both your vCenter and ESXi systems. The product interoperability matrices from the vendor is unfortunately not much detailed when comes to build version compatibilities.
The ideal solution is to always keep your product up to date, however maintenance window are sometimes a luxury, even though there is no downtime involved in some organizations. If you are stuck to a point where you need to add additional host and you are behind on updates, have a look at the build version of your current hosts and the ones available for download in the previously available versions on the VMware download site. Match the same build version, or the very previous one of unavailable (in the case where you updated with the system updater, and that version is not available with the ISO images provided on the site).
NOTE : If you end up with mismatched build number, it should still work without any major issues, but not recommended by the vendor, some glitches may occur. Update everything as soon as you can under such situation.