"Unable to show a valid ticket for your entire journey" may well suffice to satisfy the 2018 Penalty Fares Regulations' requirement that the recipient has to be provided with “an explanation of why the person is being charged a penalty fare” in writing. It might be interesting to see how Penalty Services respond to an appeal on the basis that the OP held tickets for a journey from Deansgate to Leyland but was not provided with a written explanation of why either/both of those tickets was not regarded as being valid.
I think it is vital to establish whether there was any exchange between the OP and the official as to the destination station to which the penalty fare was to authorise travel. If there wasn't then the default provision in Regulation 5(5A) of the 2018 Regulations applied, and a PF issued at Lostock should have authorised travel only as far as Horwich Parkway (assuming that was the next scheduled call for the train in question). So the question to be answered is whether
@AndroidAsh specified Buckshaw Parkway as the destination to which the penalty fare was to authorise travel (or agreed to a suggestion from the official to that effect). If not, and that destination was imposed without choice then, as
@furlong says, the PF was invalid. Any appeal on this ground should spell out that the PF was not “charged in accordance with the requirements of these Regulations” (i.e. the Railways (Penalty Fares) Regulations 2018) - see Regulation 16(2) of those Regulations.
The fact that the OP already held a valid ticket for the Buckshaw Parkway-Leyland leg of the journey makes no difference to Regulation 5(5A)'s requirement that if the PF is to authorise travel beyond the train's next scheduled call, this must be preceded by the traveller 'specifying' a different destination station at which the train is due to call and the 'collector' deciding that it is reasonable to issue a PF to that destination. If that two-stage process hasn't been followed then the PF has not been issued in accordance with the Regulations' requirements.