Hi,
First of all, the error you've shared is because the "openMenu" function does not exist in the related component.ts file and you cannot use "onclick" in Angular, and it's not related to the issue you're asking about. When it's come to the routing issue, you can follow the walkthrough below.
Since it's a static HTML file you can't define it as an initial path in Router Module since it requires to address a component or a module to be loaded. In this case, the best approach would be to convert the static HTML into an Angular component.
For example,
1- Use "ng g c landingComponent" command, it will generate a landing-component folder containing a landing.component.ts
2- Import it to the top-level module where the routing happens.
3- Replace the initial route (dashboard) with your landing.component
For more info please visit Angular documentation (
https://angular.io/guide/router) it's just how Angular works and it's not related to PrimeNG or any templates.
Regards