The incident happened at about 9.40pm yesterday (Monday, December 16) outside Jynx nightclub on Witham Road.
Two men approached a member of staff and demanded cash while brandishing what appeared to be a handgun.
The victim refused to hand over cash and the offenders made off empty-handed down Witham Road towards Sandy Lane shopping centre.
Det Sgt Diane Higham, of South CID, said: “This must have been a terrifying experience for the victim and although they were fortunately not physically injured it is concerning that the offenders were at least prepared to carry a firearm.
"I would appeal to anyone who has any information to get in touch.”
The first offender is described as 5ft 9in tall, medium build wearing black clothing. He spoke with a local accent.
The second offender is described as 6ft tall, slim, wearing black clothing.
Both men had their faces partially covered.
Anyone with information should contact police on 101 quoting log number 1373 of December 16.
Alternatively, information can be reported anonymously via independent charity Crimestoppers UK on 0800 555 111.