note, that you can enter whatever you like as the name - there is no need to enter your real name.
in general, enforced name or pseudonym is on purpose. see Android v1.46.5 forces a profile name · Issue #3160 · deltachat/deltachat-android · GitHub for some reasoning:
the profile name is the most important information others see when you contact them, it is not nice if that is blank for lots of users.
this is not only a personal decision: it would make groups much harder to read and to follow for all participants.
this is even more true when considering chatmail and randomised email addresses.
note that you can enter whatever you like here, there is no need to enter you real name. just something. at best somethings that help ppl to realise who you are, of course.
for these reasons, we enforce setting a name during onboarding
EDIT: the email address in Delta Chat is far more tuned down than in classic email programs. not specifying a name is comparable with WhatsApp usage with ppl not being in the address book - you end up in guessing who is who … same would be with chatmail addresses
so, the name is really the one data point that is reliably visible to others.
on later editing, enforcing is done on android only, iOS and desktop have issues for that
as an alternative, if you really want to set your display name to be just your email address, which is not recommended, you can set the name to your email address that way the field will not be empty