Answer from @haci, converted from comment:
You should run your command within that directory (the directory where the fastq file is), this does not seem to be the case as far as I see. Alternatively you can provide the absolute path of your fastq file.
cd into the directory where the fastq file is (you should be able to see your file of interest when you type
ls) and then run the command in your question:
seqtk seq -a KF0124_1_1.fastq > KF0124_1_1.fasta
Alternatively you can provide the absolute path of your fastq file:
seqtk seq -a ./genome_diirt/KF0124_1_1.fastq > ./genome_diirt/KF0124_1_1.fasta