Correct, the tile next to Pergamon would generate a three-way canal, that is not allowed there.
Same happened to me in the example I was commenting before that I tought was due to reefs: here, I planned to build a canal in the strait next to Jeonju, but saltpeter appeared in one tile (and cannot be harvested to put a district there), and the second tile in the strait is also next to Jeonju, so it would generate a 3-way ccanal and is not an aceptable tile. Unfortunately, I also placed the aqueduct in a tile that would have allowed me to do a 2-tile canal using Jeonju city center...
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However, from the same game, here Jinju has a channel going directly into a reef tile (it is true that there are "free" sea tiles next to it, but the channel connects right to the reef tile), so I asume reefs are OK as canal enter-exits, but it was unfortunate the saltpeter reservoir blocked channel placement on the previous example.
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