As has been confirmed pop ups is not currently possible.
However you can use a variety of different cards in the UX in the meantime to show warning messages based on a predetermined criteria.
KPI or Field cards would be especially effective in showing written messages while you could also use picture cards to display visual alerts if certain formula driven conditions are met.
Chris Heathcote
Bedford Consulting
Gold Partner and Regional Partner of the Year 2021, EMEA