When adding or updating a CNAME under Admin > Landing Pages as described here https://docs.marketo.com/display/public/DOCS/Customize+Your+Landing+Page+URLs+with+a+CNAME, you receive an error message saying “Invalid CNAME for Subscription.”
Check your CNAME in a tool such as https://mxtoolbox.com/ and see what it is pointing to. It needs to point to [ACCOUNT STRING].mktoweb.com. (Account String can be found under Admin > Landing Pages > Settings > Account String.) If it is not currently pointed to this, you’ll need to ask your IT department to update your CNAME to point to the correct place before you can add it to Marketo.