Error X11 module cannot be loaded

[Nifaste]

Hello,
I'm running alignqc with python version 2.7 and R version 3.4.1
I have the following error message during .png creation for 2 R scripts (plot_base_error_context.r , and /plot_alignment_errors.r)

Error in png(args[2]) : X11 module cannot be loaded
In addition: Warning message:
In png(args[2]) :
  unable to load shared object '/opt/conda/envs/alignqc/lib/R/modules//R_X11.so':
  libXt.so.6: cannot open shared object file: No such file or directory
Execution halted

I don't have this problem with png output from plot_exon_distro.r or plot_chr_depth.r
Could it come from R version I am using ?
Thanks in advance for your help