The connect tool is used to connect tools to one or many data points. A specific data point can be connected to none, one or many tools at the same time. A specific tool can be connected to none, one or many data points at the same time. The control plan, or control plan template, must be in the Created status. At least one tool must be defined in the Define tab.

Those may be the possible root causes for this issue. 

Also, make sure that the  Manufacturing Tools to be used in any Control Plan are “enabled for control plan” and that the validity is “Active”.

Thank you @Elath @Peter Lundgren for the answers. Now I am clear on the concern.