There's code to prevent the equipping of weapons flagged with the Power Armor flag unless the PC meets the requirements for the function pc.canUsePowerArmorWeapon() to return true. The same check is also performed to keep the PC from actually attacking with the weapon if they somehow equipped it anyway. Wearing armor with the Power Armor flag will presumably cause this function to return true, as will having sufficient Physique or having the Bigger Guns perk. I can't confirm this because I can't find the code for the function itself, since Github's search system is not ideal; all I could find other than the code defining its usage is that Chaurmine overrides it to true for himself.
If you try to equip an armor with the power armor flag you get the same message, you're not strong enough to equip that armor.
As far as the code goes, it only works with the big guns perk and 40+ physique:
public function canUsePowerArmorWeapon() : Boolean
{
return this.inPowerArmor() || this.physique() >= 40 || this.hasPerk("Bigger Guns");
}
Because the this.inpowerarmor is set to false by default.
public function inPowerArmor() : Boolean
{
return false;
}