Serum is asking for a serial, but I can't find mine.
If you purchased Serum from this website: You should find your Serum serial number in "your account" section of the website. If you don't see any purchases there, most likely you purchased under another account/email address. Use the contact form and please include as much info as possible about email(s) used and a date and time of purchase if possible.
If you are leasing Serum from Splice, see this related Rent-to-own article.