I hope you're doing great.
As per my understanding, this could be because the file you're trying to delete is downloaded to some other (download) location of your device; it must have another way of deleting it, depending on which mobile device you're using.
Also, one more reason could be that the file you're trying to delete is a shared file, to delete any shared file, you need to unshare the file first.
For more information, you can refer to these articles: -
Let me know if this works.
I download a read only files. I then can't rename nor delete. Pls help.