probleme d'instalation

it's simpler to temporarily disconnect your computer from the internet and restart you cs5, cs5.5 or cs6 activation.  (if you're trying to activate anything else, what program and version you're trying to activate?)

you can then select offline activatation and procede with activation.

otherwise you will have to trouble shoot the more complex problem of why your computer is failing to connect to adobe's secure servers. that can be caused by anything from an older os to a firewall and a lot other things.

let us know which option you want to try.  again, proceding with offline activation is the easiest.

Photoshop is by subscription only and does not use a serial number

Do you mean Photoshop Elements?

What is your computer and operating system?

TLS 1.2 is now required to connect https://helpx.adobe.com/x-productkb/multi/eol-tls-support.html

-test your browser https://clienttest.ssllabs.com:8443/ssltest/viewMyClient.html
-http://helpx.adobe.com/x-productkb/policy-pricing/activation-network-issues.html
-https://helpx.adobe.com/download-install/kb/unable-to-reach-adobe-servers.html

Moved to the Download & Install forum from Photoshop ecosystem