I'm sure about the exact commands that will work for your data, but I know the the direction we should go.
Let's work backwards.
Argument to parameter ‘data’ is not a subtype of SampleData[JoinedSequencesWithQuality | PairedEndSequencesWithQuality | SequencesWithQuality].
OK, so we need to use one of these types. Given that you only have forward reads, I think
SampleData[SequencesWithQuality]
is probably the best option.
So now that we know the desired data type of --i-data, we can make this file again, now with the proper data type.
I think qiime cutadapt demux-single will output an artifact with SampleData[SequencesWithQuality]...
Wait! How did you make untrimmed.qza? If you have not tried cutadapt demux-single yet, now it a great time to try it, because I think it will make the exact data type you are looking for.
Edit: Have you discovered the q2-cutadapt community tutorial? You should totally check that out!
Colin