You can install Customer Portal in the Salesforce through following modes:
– Salesforce Classic
– Salesforce Lightning
Salesforce Classic
To install the Customer Portal using the Salesforce Classic mode, switch to Salesforce Classic mode.
To switch the mode, navigate to Admin -> Switch on Salesforce Classic.
Salesforce Lightning
To install the app using the Salesforce Lightning mode, switch to Salesforce Lightning mode.
To switch the mode, navigate to Admin -> Switch on Salesforce Lightning Experience.