The problem is not on the input, but on the output side!
The block diagram of the PAM8019 suggest completely independently running PWM modulators for the left and right channel. (they don't necessarily actually are independently free-running. It's just that the datasheet doesn't make any statements that they'd be linked, or linkable, at all. )
That means you cannot in general directly connect the output bridges, because that might lead to shorts. (look at the diagram on page 2 of the datasheet. You'll notice the Push-pull stages. It's perfectly possible that if you connect two outputs together, one push-pull stage will connect the output to VDD, and the other to ground. Now you have a large current flowing from VDD to the first output pin, into the other output pin, into ground. That will make the chip very sad.)
So, if anything, you'd need to have an output filtering state for both channels, and then add them up. But that directly works against the filterless design. So,
Because of component shortage, I have to use the following audio amplifier : PAM8019
You will have to use two speakers, or a different amplifier IC. I'd tend towards the latter.
Alternatively, you can just ignore all outputs for the right channel, and just connect your DAC to the left channel.
But: the DAC you chose already contains, just like your amplifier, a differential output stage. That's annoying, because your amplifier has a non-differential input. The datasheet says the output has "differential output option", but doesn't say how to disable it, so it seems it's always differential.
So, honestly, wrong DAC for this amplifier, wrong amplifier for this kind of job.