default generated value is longer than what support most of the dns provider.
as a result you have to separate it in multiple field if the dns provider support it.
if you check the generated value of a less than 255 characters you will see that
default._domainkey IN TXT "v=DKIM1;p=CUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUTMEEEEEEEEEEEEEEEEE;t=y"
@ IN SPF "v=spf1 mx a -all"
@ IN TXT "v=spf1 mx a -all"
_dmarc IN TXT "v=DMARC1; p=none; adkim=s; aspf=r; rua=mailto:dmarc-feedback@mydomain.com; pct=100"
as the default key is longer than 255 the result is :
default._domainkey IN TXT "v=DKIM1;p=CUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUT""MEEEEEEEEEEEEEEEEE;t=y"
@ IN SPF "v=spf1 mx a -all"
@ IN TXT "v=spf1 mx a -all"
_dmarc IN TXT "v=DMARC1; p=none; adkim=s; aspf=r; rua=mailto:dmarc-feedback@mydomain.com; pct=100"
see the long chain is separated in two smaller one between " without newline.
if your dns provider does not offer plain text fill of zones, you might need to refer to his documentation on how to split your chain in their fields.
if they do not support long chain, then you need to modify sme config to create smaller keys, which are weaker
You might be lucky and your provider allow longer than 255 characters in their field and you simply pasted the whole chain with the "" in the middle, just remove them and it will work.