It's always useful and better to auto-assign a journey but in case if the journey needs to be assigned manually, you can very well do so.
Please follow the steps.
You may have to select the "People" section from the options on the home page.
Once you are on the "People" Section, you may now search for the employee/candidate you want to assign the journey by selecting the option "Search by name or email" under “FILTER BY”.
Once the candidate is located you may now select the candidate/employee by clicking the Name, then select Edit Profile, Job details.
Now, under "Onboarding details", "Journey", "Journeys not yet assigned" you may select the option "Assign Journey" to assign the journey manually from the existing journeys.