Error:Could not open individual file

When I ran the following command to Fst calculation

vcftools --gzvcf All_samples.vcf.gz --weir-fst-pop Pop1_list.txt --weir-fst-pop Pop2_list.txt --fst-window-size 50000 --fst-window-step 25000 --out pop1_pop2_50_25.windowed.weir.txt

I get this error

 Error:Could not open individual file: Pop1_list.txt

format of Pop1_list.txt is one individual per line like

ind1
ind2
ind3

what is wrong?