I had the same issue with the location of the directory for the parts library.
The problem is in the path it self.
/home/mypc/Documentos/cad/partlibrary is the problem
Reduce the directory by one subdirectory /home/mypc/Documentos/cad this should correct the problem.
If it does not then you need to reduce it down to a single directory below your home directory as such /home/mypc/cad
Another thing is - is this the absolute path, please make sure it is by going to the command line and cd'ing to the directory and then use pwd, you then can copy this in the path box in LibreCAD.
I hope this helps solve the problem.
When you have the will to always learn there is nothing that is impossible....The day you stop learning is the day you need to change your profession.