The modal verb 'could' is always followed by the base form of the main verb, not -ing or -ed forms. 'Make an effort' is a fixed expression meaning to try hard. The clause structure 'If we could all make an effort' requires the infinitive form 'make'.
After the modal could (in could all make), the verb must appear in its base form, so make is correct rather than the past tense made, the third-person singular makes, or the gerund making. Modal verbs are always followed by the bare infinitive.