Installed SeedProd plugin on a WordPress installation, however, error 405 is thrown when I try to add a new landing page using SeedProd.
Hello @olutayo_oyawale 
and welcome to Kinsta Community!
First of all, I’ve never heard/used that SeedProd wordpress plugin before and so have no experiences with it at all.
That error in question could be possibly due to a conflict or something with the other plugins (or theme) enabled on your WordPress site. I would suggest to try to disable other plugins and only keep or enable that SeedProd and its required/dependent plugins and see if you’re still getting the same error or not when you try to add that new landing page.
If it works just fine, then you can try to re-enable 1 by 1 the other plugins you’ve disabled before and re-test the same, until you find the culprit (to see which plugin that may be causing the error/conflict).
If none of those other plugins are causing the issue, then it’s worth to change the WP theme to the default WP theme and re-try.
You may also want to check our blog post here for that 405 error.
Also, if you host your WordPress site on our hosting server, you can reach out to our support team to check/assist you with that 405 error on your WordPress site (if something can be found in the site’s access or error logs on the server).
Best regards,
Agus