Downloading

So whatever is in the Download Folder can be deleted juts by Drag and Drop to Trash and then Empty Trash. This will open up space on the Internal Drive.

Barring that, re-boot into Safe Mode - Shift - key immediately at startup. This will do a Repair Disk, clear Cache files and only load required Apple Software to load the computer. So, it loads slowly - Normal. Once logged in >> the computer will look and behave differently - also Normal. Try drag and drop the contents of the download folder to Trash and empty. Stay in Safe Mode for about 5 - 7 minutes and re-start in Normal mode.

Starting t look like Not Enough Empty Space issue thus nothing is downloaded even if one gives permission it just can not.

Downloading by what means? Think there was a Security Update for Mojave in December 2010 which may have upgraded Safari also. When in Safari and attempt to download a file, there should be a notification in Safari able Allow Downloads from this site. If one choses No then it will not, if choose Yes, it will.