Usually this question arises when users are not sure on whether they have Analytics for Target enabled/provisioned for their account or not.
First of all, you need to get provisioned for A4T. It is not a click and go feature that can be enabled from within the Target user interface. You can ask your Account Manager or consultant to get you provisioned for A4T. Once the provisioning for A4T is complete then you can follow the below steps in order to create and A/B activity using Analytics:
Before a report suite can be selected when defining an Analytics activity, you need both an Analytics user account and a Target user account. Your user accounts must be configured as described in the following sections in this doc: https://marketing.adobe.com/resources/help/en_US/target/a4t/c_account_reqs.html
Once you are a member of one or more Marketing Cloud groups that have access to Analytics and Target and you have access to all report suites, You should see the option to create an A/B test using Analytics under Create Activity.
Sometimes there could be provisioning issues, so please check if the A4T is provisioned correctly.