You have wired two terminals that are different types, such as a numeric control to a Boolean indicator, and LabVIEW cannot convert them to the appropriate type.

To view detailed information about the types, open the Context Help window and move the cursor over the broken wire.

To correct this error, change one of the terminals or insert the appropriate conversion or transform function. For example, in the VI show above, you can right-click the object on the front panel and select Replace from the shortcut menu to change the object to a more appropriate type. You also can select the area of the block diagram that contains the broken wire, select Edit»Remove Broken Wires from Selection, and rewire the terminal.