You haven't shown us the back page of the Fixed Penalty Offer, but I suspect that Step 2 of the instructions tells you to submit your driving licence details as well as paying £100. Did you do that?
Either that or, if you did submit them, the police say they did not receive them. That will probably be the reason why you have been prosecuted.
You have two choices:
You can plead guilty to the charge and in the "mitigation" section of your response, explain that you failed to send your licence details (if you did fail) or explain that you did and they must have not been received. As a result, ask the court if they would sentence you at the fixed penalty equivalent (there is guidance that suggests this might be done if the court thinks fit).
Or, if you definitely did send your details, you can plead not guilty and defend the charge using s76(2) of the Road Traffic Offenders' Act. This says that no proceedings shall be taken against a defendant who has complied with the requirements of a Conditional Offer of a Fixed Penalty. Before you consider this option, it would be a good idea to check that you can prove you submitted your details.
