Everything is in the title, vSphere 5.5 is dead.
As described here by VMware end of support will be september 19, 2018.
What should you do?
Upgrade to vSphere 6.7 is THE choice i think, if you’re not confident with the last version of vSphere 6.7, you can go to vSphere 6.5.
Not so much to say, the thing to notice is that “Windows Client” is no more available, only web client.
HTML5 client is really handsome, Flex client aka Flash client is also still available, but new features will only be available trough the HTML one, so go directly with this one.
If your vCenter is in 5.5 and hosted on a Windows VM, you can now go to vCSA version, in fact you must go for it, update manager is now packaged on it.