I have a symlink /Library/WebServer/Documents/2Tlnk to /Volumes/2TMacBook/lnk1 which is a directory with an index.html file in it. 2TMacBook is an external drive, not a separate MacBook computer. I get a 403 (You don't have permission to access this resource.) error when I try to access the page. If I bypass Apache and direct my browser to file:///Volumes/2TMacBook/lnk1/index.html or file:///Library/WebServer/Documents/2Tlnk/index.html I see the page. And file:///Volumes/2TMacBook/lnk1 gives me the directory listing.
My Mac is an iMac, about 1 1/2 years old, running macOS Big Sur version 11.3. My Apache version is 2.4.46 built Feb 28, 2021.
Similar symbolic links to directories on the internal disk in my Mac work just fine, both in /Library/WebServer/Documents/, to /Library/WebServer/Documents/test and to ~/test. I have turned on ownership on the External drive, 2TMacBook. I have set the group ownership to _www on the directory that is the target of the symlink and the index.html file within it. I have, at this point, given read-write-execute permission to everyone on 2TMacBook, lnk1, and index.html in lnk1, and the same permissions where _www is the group.
I have also tried the Apache Alias mechanism. Again, it works for internal drive directories but not for my lnk1 directory on the 2TMacBook external drive.
Just in case there was something funny with 2TMacBook, I tried everything also on another external drive. The 2TMacBook drive is formatted as Mac OS Extended (Journaled) and the other drive I tried is formatted as APFS.
At the moment I can't think of anything else to try. Any help would be appreciated. I used an external drive for extra web page storage a year or two ago. It worked then and it sure was handy. I'd like to have it work again.
This question was answered in the comments. I'll add an answer so it's clear to future readers that there is an answer.
The answer is to give /sbin/httpd "Full Disk Access" in "System Preferences" -> "Security & Privacy" -> "Privacy" tab.