Hi Louein,
I wasn't able to find that detail in our documentation, and we actually no longer support billing SSIDs/prepaid cards directly through the Meraki Splash page. A read-only admin should NOT be able to make any changes whatsoever, so if you are able to create pin codes as a read-only network or organization admin, please open a support case to have the bug documented and sent to engineering to resolve.
I was not able to replicate this behavior on my own accounts.
If you are interested in a billing/paid SSID option via splash page - these are available via our partner portals which the details of can be found here:
https://documentation.meraki.com/MR/Splash_Page/Splash_pages_with_PayPal_or_Credit_Card_Billing