As of Asterisk 19 the res_monitor module is not built by default:
res_monitor
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* This module is no longer built by default in
accordance with the Module Deprecation Policy.
If you require this functionality you will need
to enable it for building in menuselect. Note
that in the future res_monitor will be removed.
This is documented in the UPGRADE.txt file.
You will have to go into menuselect and ensure it is enabled.