What does it mean maximum referral line18 must be entered

No one can see your screen -- we do not know what "line 18" you are asking about since there are so many possible forms and schedules it could refer to.

You are posting from TurboTax Live.  You can arrange for the Live help you are paying the extra fee for with your questions from 5 a.m. to 5 p.m. Pacific time.

https://ttlc.intuit.com/questions/4124827-how-do-i-connect-with-a-tax-expert-in-turbotax-live

That's for the self employment tax.  You can defer it.  That part of the program doesn't work right.   I had trouble efiling because of it.  I had to put in zero or override it or something.  

Will these posts help?

https://ttlc.intuit.com/community/taxes/discussion/why-do-i-keep-getting-me-to-enter-my-sch-se-t-deferral-amt-thought-it-was-automatically-put-in-for/00/1845078 

https://ttlc.intuit.com/community/state-taxes/discussion/as-i-m-reviewing-my-federal-taxes-it-s-telling-me-to-fill-out-line-18-of-the-schedule-se-worksheet-i/00/1858174