The predecessors of a summary task or predecessors of an inserted project summary task must have a finish-to-start or start-to-start ...

{The predecessors of a summary task or predecessors of an inserted project summary task must have a finish-to-start or start-to-start dependency.}Select the tasks you want to link in a finish-to-start or start-to-start dependency, and then click the Link Tasks button on the Task tab. Selected summary tasks are also linked.