The name of the city/town you have stated in the form has to match the post code you have given. If you check the application form, the link under [?], http://www.posta.co.ke/postalcodes.asp, you will find it’s currently not working or it’s wrong. I believe that is the link that Google uses to search the post code for your given city/town from Posta Kenya website, and the result obtained is compared with the post code you have written on the form. If this is the case, and the link is dead, then you’d have to contact Google directly. This may take several weeks to get an answer.
The other thing I notice is that the postcode number for Nairobi GPO that you have given as 00100 should be 0 0100. That space in between zeros may make all the difference. Just keep on trying because Nairobi GPO may not be taken to be the same as just Nairobi.