We know that St. Phoebe and others were “deacons” (Romans 16:1, diakonon), but how was their ministry expressed?
Apostolic Constitutions, Book 8, Paragraph 28: “A deaconess does not bless, nor perform anything belonging to the office of presbyters or deacons, but only is to keep the doors, and to minister to the presbyters in the baptizing of women, on account of decency.”
St. Paul did not give women teaching authority, especially at Mass (1 Corinthians 14:34, 1 Timothy 2:12). Deacons must be able to proclaim the Gospel in the liturgy (Summa Theologiae, TP, Q. 67, A. 1, R. to Ob. 1), and all priests are to first be made deacons (Canon 1050). Therefore, women cannot serve in either role, and so they cannot be ordained at all.