Are both of them dripping? Can you tell if it is from the 3/4" coupling or the 15mm compression end?
I would:
remove both, rewrap the 3/4" end with PTFE, thinner at the open end and thicker towards the centre. Tighten into the mixer, the connect the 15mm WITHOUT PTFE - it should not be necessary on a clean new joint, unless you have over-tightened the connection already.
I would:
remove both, rewrap the 3/4" end with PTFE, thinner at the open end and thicker towards the centre. Tighten into the mixer, the connect the 15mm WITHOUT PTFE - it should not be necessary on a clean new joint, unless you have over-tightened the connection already.