The system prevents the merging of two records where one or both is a League Registered Player if the details are different or the match will amend the critical registration details (name, D.O.B etc.) of the LRP or the remaining record if both records are LRPs.

This is largely to prevent club administrators changing those details by matching without the knowledge of the league(s) involved, but also in case of league administrators matching records for two different registered players.

You need to check those details in both records and if they are the same player then amend so that they agree exactly (you may need to involve the Helpdesk to assist with this if it is the details for the LRP record that are incorrect) - you should then be able to match the two records; alternatively you could withdraw the league registration(s) of the player if that is the incorrect entry, make the amendment and perform any matching before resubmitting the registration

See also Why can't I amend a player's details? and Merging Player Records