Install libnpgsql-cil-dev by entering the following commands in the terminal:
sudo apt update sudo apt install libnpgsql-cil-dev
Description:
PostgreSQL connection library for Mono - devel package
Npgsql is the .NET data provider for PostgreSQL. It allows any program developed for .NET framework to access a PostgreSQL database server. It is implemented in 100% C# code. PostgreSQL versions since 9.0 are officially supported, others may work. . This package contains development files for Npgsql and will always pull in the current default version - and should be used for compilation.
Homepage: http://www.npgsql.org/
Version: 2.2.7+dfsg3-1
Section: universe/cli-mono