my apple watch 2 , always pauses when im doing exercises

In addition to the information from Ingo2711,

are you moving sufficiently to raise your heart rate

and keep it elevated?

Is the arm with the watch constantly moving?

Easing my effort or keeping my watch arm relatively motionless

(like pushing a stroller or grocery cart) have caused that

warning to come up on my watch 10.

Any chance there are tattoos beneath the sensors?

Wrist detection is enabled?

Make sure the strap is tight enough.