You can add relations to your tasks to indicate dependencies or blockages. To do this you first need to activate the task relationships feature for your project.
Click on the ( i ) button in the top bar of your project and select Manage from the menu. In the dialog that appears, switch to the Features tab. There, flip the switch next to Task Relationships.
Once you have activated the feature, you can add relationships to your tasks. Please note that if you want to add a relationship between tasks that are located in different projects, the task relationships feature needs to be activated in each respective project, or else it won't work.
How to Add a Relationship
Open a task and click on Relations in the right sidebar of the task dialog.
This will open the relationships dialog, where you can select the type of relationship as well as the task you want to connect to this one. You can use the search field to look for the task if it doesn't automatically appear in the list below the dialog.
If you choose "blocks" or "is blocked", an orange icon will appear on the task tile of the blocked task, indicating the blockage.
Other task relationships are not displayed in the task tiles at the moment, however, they are always visible inside the task dialog, where you will also find links to all connected tasks.