Below are the mentioned steps to generate solution manager key. It is mandatory to use solution manager key during installation of ecc5 and above versions of SAP
1 - Go to transaction SMSY
2 - Right click on system entry and select Create new system
3 - Enter the system name (3 chars), Product (SAP ECC), Product version (can select from the list)
4 - Save your entries
5 - Select System --> Other Configuration.
enter the SID you have created earlier. Enter the Server name or the host name.
6 - Click on generate "Installation/Upgrade"
It will show the solution manager key which should be copied to the installation wizard.
if you do not currently have Solution Manager you should click here