Planet fitness demanding payment after the account has been closed in 2019

Hi I closed my account /membership the 13th December 2019 , and this company just keeps on demanding payments from me , refer no from planetfitness :PF*** dated 13th December 2019 at 2:48pm I have tried to email them call them and eventually went to the gym that I opened the account with and still no one has come back to me . Please can you help me and stop this demands . thanks Kind regards