How do I make a form field editable based on a condition?

For a datasource-bound field, use allowEdit() on the datasource field.

boolean canEdit = salesTable.SalesStatus == SalesStatus::Backorder;

SalesTable_ds
    .object(fieldNum(SalesTable, YourField))
    .allowEdit(canEdit);

I prefer this over only enabling/disabling a single control because it applies to the bound field regardless of which control is displaying it.