Odd that Turbotax doesn't have a better answer for this. Just throwing an error with no guidance is weird. Searches turn up some information, such as:
"some tax preparation software’s — such as Taxact — will allow you to e-file as Married Filing Separately without your spouse’s SSN or ITIN... Every year, I successfully e-file tax returns for my clients even when their spouses don’t have an ITIN or SSN."
https://oandgaccounting.com/need-itin-for-wife-even-though-married-filing-separately/
