I can only agree.
'Che concordare' — ke con-cor-DA-re. The 'che' after 'non posso' creates the 'non... che' (can only) structure.
Use when agreement is so compelling that no other response is possible. The 'non... che' construction means 'can only' — there is literally no alternative but to agree.
'Non posso che' + infinitive is an Italian construction meaning 'I can only / I cannot but'. It implies the action is the only possible or appropriate response. 'Non posso che concordare' = 'I can only agree' — the evidence or argument compels agreement.
Non c'è altro da aggiungere.
There's nothing more to add.
Implies the point has been made so completely that further words are unnecessary
L'argomento è inconfutabile.
The argument is irrefutable.
'Inconfutabile' (irrefutable) — the strongest acknowledgement of an argument's validity
Non posso che darti ragione.
I can only concede you are right.
Direct application — you are given reason/right. Very formal and dignified concession.
'I fatti parlano da soli' (the facts speak for themselves) is a powerful Italian rhetorical move — it implies that arguing against established facts is irrational. In Italian culture, where oral persuasion is highly valued, deploying this phrase correctly ends most arguments.