Install libnpgsql2.2-cil by entering the following commands in the terminal:
sudo apt update sudo apt install libnpgsql2.2-cil
PostgreSQL connection library for Mono
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.