DPMA (res_pjsip_endpoint_identifier_dpma) expects a “default outbound endpoint” in PJSIP to handle DPMA-registered Digium phones.
This is normally set in pjsip.conf with an endpoint named dpma_endpoint or similar.
If you’re not using Digium phones with DPMA, you can disable the faulting module and 21.10.0 will run fine.
edit /etc/asterisk/modules.conf
add
noload => res_pjsip_endpoint_identifier_dpma.so