When using CREATE EXTENSION postgis;
I get the following error message:
ERROR: could not load library "C:/Program Files/PostgreSQL/11/lib/rtpostgis-2.5.dll": The specified module could not be found. SQL state: XX000
I Tried to create the extension both from pgAdmin 4 context-menu and SQL command as well as command-line via psql.
as per https://trac.osgeo.org/postgis/ticket/4482 - followed the procedure and it fixed the problem (for me).
When restarted pgAdmin, was able to create the postgis extension on selected database.