There are multiple ways to access Sales & Success Center from Partner Center:
In Partner Center, click on Sales > Open Sales Center.
You can access Sales & Success Center from Partner Center > Sales > Salespeople > Salesperson Login URL (top left corner of page).
Impersonate a Salesperson by clicking the impersonate icon.
Select the icon in the top right corner of Partner Center, then click "Sales & Success Center".
Navigate to an account in Partner Center under Businesses > Accounts, then select "Open In" in the top right corner, then select "Sales & Success Center".