AES-256 was added via ASTERISK-26190. Please, note it’s description, especially the last sentence: ‘When you have to go for additional suites on egress, enable those via CFLAGS’. Consequently, as of today, you cannot enable/configure AES-256 at runtime. Instead, you have to re-configure your Asterisk and compile it again:
make distclean
CFLAGS='-DENABLE_SRTP_AES_256' ./configure
make
sudo make install
If that does not work (anymore), please, report here (and create an issue report). However, if you want to have this configured at runtime, this would be a Feature Request…