Mailflow is built into Gmail, and unfortunately, there are a few Gmail configurations that are incompatible with Mailflow.
You might have turned the conversation view off because you don’t prefer Gmails grouping of threads. Mailflow requires the conversation view to be on for you to be able to add emails to projects, add tasks next to emails, or set statuses on threads.
Step 1: Click the settings icon in Gmail.
Step 2: Scroll down and make sure that the checkbox is checked.