I assume that you added arrays to SUMMER_TILESET, AUTUMN_TILESET, WINTER_TILESET, and SPRING_TILESET, which caused the error. The plugin comes with an example, but I'll explain each part here.
There are two parts for that to work, one in OUTDOOR_TILESETS and another in the season tilesets.
In the OUTDOOR_TILESETS you have to put the ID of all tilesets you use when creating a map that you want to change depending on the season. And it has to look something like this: OUTDOOR_TILESETS = [1, 2, 3, 4]. The numbers don't have to be these, but they have to follow this structure.
Then, on the part of the season tilesets, you have to put a single number. That number will be added to the ID in the OUTDOOR_TILESETS to get the ID of the corresponding seasonal tileset. Following the example I put above, it could be something like this:
OUTDOOR_TILESETS = [1, 2, 3, 4]
SUMMER_TILESET = 4
AUTUMN_TILESET = 8
WINTER_TILESET = 12
SPRING_TILESET = 0
With that configuration, the plugin will do this with a map that uses the tileset with ID 1:
Spring: 1 + 0 = 1 so tileset ID 1 will be used as the spring tileset for the map.
Summer: 1 + 4 = 5 so tileset ID 5 will be used as the summer tileset for the map.
Autumn: 1 + 8 = 9 so tileset ID 9 will be used as the autumn tileset for the map.
Winter: 1 + 12 = 13 so tileset ID 13 will be used as the winter tileset for the map.
This configuration puts together the tilesets of the same season together. If you want to put together the tilesets that will transition from one to the other (so you have the spring, summer, autumn and winter tilesets for the same maps one after the other), you have to put a configuration like this one:
OUTDOOR_TILESETS = [1, 6, 11, 16]
SUMMER_TILESET = 1
AUTUMN_TILESET = 2
WINTER_TILESET = 3
SPRING_TILESET = 4
With that configuration, the plugin will do this with a map that uses tileset with ID 1:
Summer: 1 + 1 = 2 so tileset ID 2 will be used as the summer tileset for the map.
Autumn: 1 + 2 = 3 so tileset ID 3 will be used as the autumn tileset for the map.
Winter: 1 + 3 = 4 so tileset ID 4 will be used as the winter tileset for the map.
Spring: 1 + 4 = 5 so tileset ID 5 will be used as the spring tileset for the map.
You have to make sure to put all the tilesets for the same season at the same distance from the tilesets used to create maps (which should be all of the same season unless you use 4 different tilesets for the seasons) so the plugin can change them correctly.
Hope this helps understand how that part of the plugin works.