Right now the program only supports Lemmy BE 0.18.1-rc9, but new releases will try to support new versions as they are released. The Lemmy API is changing a ton right now, but I'll try to keep up.
If you give me the instance I can confirm, but that field was added in Lemmy 0.18.3, and I suspect the instance you are trying to use doesn't support that yet. You can see the Lemmy version at the bottom of the page.
LASIM 0.2.0 only supports Lemmy 0.18.3.
LASIM 0.1.2 only supports Lemmy 0.18.2.
So you can wait for your instance to upgrade and then use LASIM 0.2.0, or you can use the older LASIM 0.1.2.
Note: if say your old instance is Lemmy 0.18.2, and the new one is Lemmy 0.18.3, you can use LASIM 0.1.2 to download, then use LASIM 0.2.0 to upload.