Hello,
I try to setup the www subdomain of my original domain hosted as GitLab Pages
(similar to Pages: 401, no permission, cannot verify domain although DNS has been set but I (AFAIK) did what was proposed as a solution)
I added a
www CNAME agile-securiy.gitlab.io.
and
_gitlab-pages-verification-code.www TXT gitlab-pages-verification-code=1f21fd16ee7212772056613ceb22af07
in my domain administration UI
But I the verification of my ownership fails in the pages setting
The DNS entries are there (and it is roughly 12h since I created the entries)
$ dig +short cname www.agile-sicherheit.de.
agile-securiy.gitlab.io.
$ dig +short txt _gitlab-pages-verification-code.www.agile-sicherheit.de
"_gitlab-pages-verification-codegitlab-pages-verification-code=1f21fd16ee7212772056613ceb22af07"
I don’t understand what I’m doing wrong?
The original page works with A and a TXT records
$ dig +short a agile-sicherheit.de.
35.185.44.232
$ dig +short txt agile-sicherheit.de.
...
"gitlab-pages-verification-code=e186c751fd4a367b198d8501b4093ce3"
Any ideas?
OMG Now I see it, classical PEBCAK.
For some reason I created a
_gitlab-pages-verification-codegitlab-pages-verification-code
instead
_gitlab-pages-verification-code
TXT entry. SO embarrassing 8-o
Ok, I corrected the error but I still can’t verify the page 
$ dig +short txt _gitlab-pages-verification-code.www.agile-sicherheit.de
"_gitlab-pages-verification-code=1f21fd16ee7212772056613ceb22af07"
$ dig www.agile-sicherheit.de
; <<>> DiG 9.10.6 <<>> www.agile-sicherheit.de
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 517
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;www.agile-sicherheit.de. IN A
;; ANSWER SECTION:
www.agile-sicherheit.de. 3295 IN CNAME agile-securiy.gitlab.io.
agile-securiy.gitlab.io. 300 IN A 35.185.44.232
;; Query time: 22 msec
;; SERVER: 192.168.2.1#53(192.168.2.1)
;; WHEN: Tue Apr 07 12:54:16 CEST 2020
;; MSG SIZE rcvd: 105
Anyone any ideas?
P.S.: I would really like to have more verbose error message than Failed to verify domain ownership
It would be nice to see what was tried (which DNS lookups) and if some keys are wrong?
Hopefully this is the final reply
After I deleted the domain and re-added it (with a new verification-code) it now works.
Now only waiting for the Let’s Encrypt certificate 
I am getting a similar problem of verification on my www sub-domain. The verification failed.
Did you delete the domain from gitlab and added it again ?