Answer:
The reason for this warning is that asymmetric cross-sections cannot be designed using the methods mentioned in 6.3.3 of EN 1993‑1‑1 or the General Method according to 6.3.4 of EN 1993‑1‑1.
You can perform the stability analysis as a cross-section design according to the second-order analysis, which is possible using the add‑on modules FE‑LTB or STEEL Warping Torsion.