IRS is pedaling as fast as they can, but the form is still in draft. PTO will make it available shortly after IRS makes it available. You are not doing anything wrong, other than trying to get a return finished this soon. If you really want to file, do so without the 2210 and IRS will assess based upon filing date and you can send a 2210 in response once the notice arrives. Filing early helps hold the penalty down.

Alternatively, have the client make an extra 1040ES payment now, being sure to mark it for 2018. I would use DirectPay https://directpay.irs.gov/directpay/payment?execution=e1s1