Wife cannot create character on my realm

[Enkidoh]

Firstly, what Membership tier for your subscription are you currently set as? In the Mogstation you have a choice of two membership tiers: 'Entry', and 'Standard'. Entry level Membership is the cheaper option but restricts you to only being able to create a single character on any one server, up to a maximum 8 characters total. If you want to create multiple characters on the same server, you need to upgrade your Membership to the more expensive 'Standard' tier which allows up to 8 characters created on the one server, to a maximum 40 in total. You can upgrade your Membership tier on the Mogstation.

If you already have the Standard tier, then it is sadly possible that Sargantanas might have indeed temporarily been locked to new character creation due to simply being 'full' at that time. If it has a New Player Creation bonus that might explain why it has suddennly filled up. The lock would probably be only temporary so you could try again at a later time (try creating a new character during 'quiet' times when there are usually less people playing, such as early/mid morning mid week). SE regularly posts updates on the Lodestone about the current server status and whether a server is 'closed' to new character creation or not which can be used as a guide, but are quickly outdated as the information is only posted every six hours or so. The current server character creation status can be found here: https://eu.finalfantasyxiv.com/lodestone/worldstatus/

[Iscah]

Firstly, what Membership tier for your subscription are you currently set as? [...l]

That should only be relevant if they're trying to share a single account - which won't be the case if she's been "recruited as a friend".