I am using ChIPseeker, however when I open the annotation source file such as Mus_GRCm38_94.gtf, it tells me that is unavailable. However, I was able to see the content of the file. I also have a link such as ftp://ftp.ebi.ac.uk/pub/databases/gencode/Gencode_mouse/release_M18/gencode.vM18.annotation.gtf.gz. I used to able to open and use it and now ChIPseeker tells me it is unavailable. I am wondering what could be the problem.
GTF datasets from this source include header lines which make the data out of specification for the type. The datatype GFF is probably currently assigned. Remove the header lines and reassign the GTF datatype. How-to is covered in this prior Q&A: