Lumen/Nanite only work with deferred rendering. Along with a myriad of other features.
The forward renderer is faster, consumes less gpu memory, allows you to use MSAA and alpha2coverage masked translucency, at the cost of having limited feature support.
The deferred renderer is slower, consumes more gpu memory, but gives you access to the vast majority of Unreals features (other than A2C and MSAA)