Hi @ejacksonBPT

I do not believe there is functionality within the sheet to lock cells until a certain cell is populated.

Some initial workarounds I can recommend are:
1) Smartsheet Forms that have required fields and then will automatically input the data into the corresponding columns.
2) Conditional formatting that highlights the other cells red unless a particular column/cell is filled alerting users that they shouldn't fill out other information until the desired cell is filled. It serves as a warning but is still prone to human error.
3) Some combination of locked rows, columns, and automations to unlock rows. The only issue I foresee with this is that even if you unlock a row, any locked columns will still be un-editable by anyone else other than sheet owners and admins.

A quick glance through product ideas and I couldn't find exactly what you were referencing but it has been discussed on forums before. Most of the discussion surrounding mandatory information comes from Forms, Approvals, and Automations. Might not be a bad idea to submit in Product Ideas!

Best,
Dan