It seems there is a problem with the fields on this competition: www.mumsnet.com/competitions/2018/win-the-modern-italian-plus-100-voucher-john-lewis
The error message I received seems to suggest an email address is expected in the Mumsnet username field. I have blacked out my email address on the attached screen capture.
It is probably just that the field has be initialised incorrectly to expect an email address format.