I recommend getting in touch with the authors of that data to understand where the barcodes are in the fastq files and if they have recommendations on how these data need to be prepared for analysis in QIIME 2. The txt file is just a sample metadata file that lists which barcodes correspond to each sample, but are not the barcode reads themselves.
The barcodes are probably included at the 5' ends of the sequences in the fastq file, in which case you can follow the q2-cutadapt tutorial to import and demultiplex these reads.