Answer
If two activities selected for linking are overlapping on the timescale (e.g. the start of a successor starts before its predecessor finishes), an error message will occur. This is because there’s no way to know whether the start of the predecessor should be linked to the start of the successor or if the finish of the predecessor should be linked to the finish of the successor. To avoid such ambiguity, place a start embedded node on the predecessor (to create a start-to-start relationship) or place a finish embedded node on the successor (to create a finish-to-finish relationship). The two can now be linked together by selecting the embed and the opposing activity and clicking the Link icon.