This message indicates that Aktiia was not able to acquire an accurate measurement during the stated time slot. This is usually due to a person being active for most of the 2 hours. To ensure accuracy, Aktiia only takes measurements when a person is at rest.
Therefore, if you perform physical activities or move your wrist, the Aktiia bracelet will not accept any measure. Other reasons for missing data points might be the bracelet is worn too tight or too loose, there is a decreased or flow of blood to your wrist (this happens, for instance, when you are feeling cold).