reactivate adobe xi

Acrobat 11 is End of Life. 

For security & compatibility reasons, you need to replace it with modern equivalents. 

MODERN OPTIONS:
==============
FREE Acrobat Reader
https://www.adobe.com/acrobat/pdf-reader.html

FREE Online PDF Editor
https://www.adobe.com/acrobat/online/pdf-editor.html\

Paid Acrobat (Standard, Pro or Studio)
https://www.adobe.com/acrobat/pricing/compare-versions.html

Hope that helps. 

What old system... exactly what is the error message?

Is that old system Windows 7?

IF win7...

TLS 1.2 is now required to connect https://helpx.adobe.com/x-productkb/multi/eol-tls-support.html
-If you have Windows 7 be sure to do the EASYFIX.MSI in step 2 of the above link
-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
-https://helpx.adobe.com/creative-cloud/kb/troubleshoot-download-install-logs.html#error113
-https://docs.microsoft.com/en-us/mem/configmgr/core/plan-design/security/enable-tls-1-2

what problem do you see when activating?