The reason for this warning is that tapered cross-sections cannot be designed using the methods given in Section 6.3.3 of EN 1993‑1‑1. The RF‑/STEEL EC3 add-on module designs such cross-sections using General Method 6.3.4 of EN 1993‑1‑1. However, this method is only allowed for loading in the support direction; that is, without the transverse bending moment. Therefore, the warning appears.
You can perform a stability analysis according to the second-order theory as a cross-section design, which is possible with the RF‑/FE‑LTB or RF‑/STEEL Warping Torsion add‑on modules.
As an alternative, you can neglect the transverse bending in the filter options of the "Details" dialog box or the "Stability" tab, or activate the adapted method according to Naumes in the "National Annex" dialog box, which enables transverse bending.