@Dyiing So I moved the schematics directory to the root of my .minecraft folder and then try making a new schematic with //schematics save sugarcanefarm. I moved it from .minecraft/config/worldedit/schematics (which is the default location minecraft saves these files) to the recently moved .minecraft/schematics directory. I then closed minecraft and check the properties tab of the file in question (which can be seen here). Reopening minecraft and running the @build command again still gives the same error.
P.S, removing .schem from the text box on that property page causes the file type to change to a generic file format, so it definitely does not have a double file extension in the name