If it's a N1SDT form, dated 15 June (but you say "received", so maybe dated 10 June?) then it needs a response within 14+5 days of the date on the form, best case by 6 July.
This response is either an Acknowledgement of Service or a defence, and can be submitted online using MCOL.
If an AoS, a further 14 days are allowed for a defence.
Missing the deadline allows the claimant to apply for a default judgement so this needs to be attended to immediately.
The first thing to do is to use the details on the form to log on to MCOL and to check the status. Then, if there is not already a default judgement recorded, file the AoS immediately. Then note the deadline for submitting a defence, and construct and submit one in time.