Over the years it comes and goes and sometimes eBay take it away from sellers that sell over a certain amount.

 

After listing select revise and see if it appears then.

If you are listing with Buy It Now, the option disappeared in February.

eBay has been known to do weird things on Fridays (US time), usually it fixes itself after waiting a bit and listing/revising again.

Unless of course you have gone over the income threshold (which AFAIK is/was $12000 in 12 months) where eBay will suddenly remove the option to automatically relist (except for GTC where it's guaranteed at the cost of free listings/insertion fees).