Want to use SharePoint Online SPFX Solution to SharePoint 2019 On Premises

DJoshi2302 

First of all https://docs.microsoft.com/en-us/sharepoint/dev/spfx/release-1.12.0 

Create a normal project to SP 2019 this will downgrade to 14.1.

See what you have to change, like typescript versions.

There is a good tool but I don't know if you have an option to downgrade, if not try to request a PR

https://pnp.github.io/cli-microsoft365/