Short answer: new Google Workspace mailbox allotment is not currently available as a self-serve purchase. If your plan already includes Google Workspace mailboxes, you can still create and use them normally — this only affects buying additional Google Workspace allotment beyond what your plan includes.
Why the Google option might show 0 available
If Google shows greyed out or 0 available on the mailbox-creation screen when you expected to be able to create Google Workspace mailboxes, it's usually one of these — not a bug or a network issue:
Your current plan is SMTP-only and has no Google Workspace quota to draw from.
You just checked out and the quota hasn't applied yet — wait a few minutes and refresh.
The specific domain you're trying to use isn't Google-eligible (see Why are some of my domains not eligible for Google Workspace?).
If you paid for a plan expecting Google Workspace mailboxes
If you signed up or added an add-on expecting to get Google Workspace mailboxes and it turns out that allotment isn't currently purchasable, contact support with your account details. We can confirm what your plan actually includes and help sort out billing if you were charged for something you can't use — don't assume you're stuck with a charge you can't get value from.
Already have Google Workspace mailboxes and want to switch them to SMTP, or vice versa?
Not sure which situation applies to you, or whether you already have unused Google Workspace slots on your account? Contact support and we'll help you check.