Punch-in is the action using which one makes progress on a KR or an objective. As mentioned in concepts about OKRs, an objective can have associated KRs and/or aligned objectives that are contributing to its progress.
In such a case, the progress of the objective happens through these KRs and/or contributing objectives. Thus, in that case, manual punch-in on the objective would be disabled. The only way to update its progress is through progress on KRs and/or contributing objectives.
Do note that, if the weightage of KRs & contributing objectives is 0, then that means KRs & contributing objectives progress isn't in any way impacting the current objective. In this case, punch-in action would still be available on the objective despite having KRs & contributing objectives.