To get rid of the instability warnings you may change the supports as noted below
SUPPORTS
*1 2 PINNED
1 2 FIXED BUT MZ
In design parameter, you do not need the following parameter as you do not have tapered section
TAPER 0 ALL
MAIN 0 ALL is not required as slenderness is checked by default anyway.
Your beam member 3 is failing in slenderness. If you do not want to check slenderness for the beam, you may set MAIN 1 for that beam. However once I by passed the slenderness check I found that the member is failing in strength check as well. So seems like you need a bigger section than what you are using now.
But even before you worry about the design part, there is another very important thing you need to check and that is the lateral displacement. You have high X displacement of 358 mm for lateral load cases 3 and 4. This is because you have pinned supports at the base. Check whether this amount of displacement is acceptable or not. If not, you may have to increase the structure's lateral stiffness. Using stronger columns, fixing the bases, adding bracing etc. are some of the options.
SUPPORTS
*1 2 PINNED
1 2 FIXED BUT MZ
In design parameter, you do not need the following parameter as you do not have tapered section
TAPER 0 ALL
MAIN 0 ALL is not required as slenderness is checked by default anyway.
Your beam member 3 is failing in slenderness. If you do not want to check slenderness for the beam, you may set MAIN 1 for that beam. However once I by passed the slenderness check I found that the member is failing in strength check as well. So seems like you need a bigger section than what you are using now.
But even before you worry about the design part, there is another very important thing you need to check and that is the lateral displacement. You have high X displacement of 358 mm for lateral load cases 3 and 4. This is because you have pinned supports at the base. Check whether this amount of displacement is acceptable or not. If not, you may have to increase the structure's lateral stiffness. Using stronger columns, fixing the bases, adding bracing etc. are some of the options.