Under Web Services > IP Restrictions, you would like to add a lot of IP addresses to whitelist for IP-based API access. The steps are described here: https://docs.marketo.com/display/public/DOCS/Create+a+Whitelist+for+IP-Based+API+Access
Even though this is not mentioned in the doc, there is a character limit for the string value. The limit is 2,000. However, the way it is stored in the backend is a bit different from UI. Therefore, it is possible that you have exceeded the limit even though it doesn't look like so.