pg_textsearch isn't relocatable. You must add it to shared_preload_libraries and restart the server before you can create the extension.
Add
pg_textsearchto theshared_preload_librariesparameter, either inpostgresql.confor withALTER SYSTEM:ALTER SYSTEM SET shared_preload_libraries = 'pg_textsearch';
If
shared_preload_librariesalready lists other extensions, appendpg_textsearchto the list rather than overwriting it. The order doesn't matter.Restart Postgres for the change to take effect.
Create the
pg_textsearchextension in your database:CREATE EXTENSION pg_textsearch;
Confirm the extension is installed:
SELECT extname, extversion FROM pg_extension WHERE extname = 'pg_textsearch';