Dear Corrina.
To be sure to be clear. My issue is not to create a subdomain in 1&1. It is easy.
My issue it to make my network wordpress sub site in subdomain work.
network site : blog3.gcm-ecriture.com : works
subsite : test.blog3.gcm-ecriture.com : does not work
Thanks a lot dear Corrinarusso.
I am not using CloudFlare but Ionos 1&1.
I have called one host “expert” yesterday and he could not help me.
I have a wildcard SSL *.gcm-ecriture.com
I will see if I can use Plesk.
I have suggested a french translation.