Projects tasks can be assigned to the client contacts. For this, please enable the following configurations: 

• Go to the Settings>Client portal>Client permissions 
• Enable the option for Client Can View Tasks by checking the box.

Then go to the project details page and add your client contacts. 

• On the project details page, click on the Overview tab. 
• Find the Client contacts section and add the client contacts. 

After doing the above things, you can assign the tasks to the client contacts.