Do you have a glyph titled “germandbls.calt”? You should have duplicated the “Germandbls” glyph and renamed it “germandbls.calt” – the case is important in naming your glyphs.
Edit: for extra clarity I am attaching my “germandbls” glyphs. You can see I have two uppercase ones, one titled “Germandbls” and one titled “germandbls.calt”. The feature you are trying to write requires the .calt glyph to be in your font.
Great, but you are still missing a glyph named “germandbls.calt” – that is what the error is telling you. Look at my screenshot again and read the names carefully. The capital glyph is duplicated, the name is lowercase and has the suffix “.calt”. The suffix of “.calt” is crucial for your feature to compile correctly.