Erik Kostson
Ansys Employee

As I said for isotropic we only need EX adn PRXY - EY and EZ , ... are not need.

See the points on how you could aid convergence:
FIrst I would recommend to include an imperfection (not only via the prestress), so say a half sway (sinus along beam), or one buckling mode that you expect (there are many posts on these - see upgeom command). This is very important for a structure like this otherwise numerical noise (molinear iterations) will trigger buckling which we do not want.

So you would like to use perhaps stabilization (see stabilization command in apdl manual), and/or transient with quasi static option (antype,4 and tintp,quas). That is the best one can do and that normally sorts this type of instability problems out.

Also use two load steps, one with the prestress, and later followed by the buckling load.

As a last resort try the implicit method (SEMIIMPLICIT command), and if that does not work also then use Explicit (LS-Dyna).
Finally I would recommend doing this mechanical not in apdl as it is much easier to troubleshoot.