Diablo II Resurrected - Battle.net connection problem
That is because D2: Ressurected is not released yet.
It releases on the 23rd of September. The servers will not be available before then.
But why is it on the store and can be booted? There is a Beta coming this weekend, starting on the 20th.
Following the Early Access weekend, on August 20 at 10:00 a.m. PDT, the Open Beta weekend will be open to everyone on a supported platform.
https://news.blizzard.com/en-us/diablo2/23688836/play-diablo-ii-resurrected-during-the-early-access-and-open-beta