It may be best to set the meter aside for now then and write up symptoms in gory detail, they'll be needed for a refund or exchange. Like so:
Summary: charged battery seems dead because bike display doesn't come on.
Blue bike : battery 36V 15Ah serial 123, controller 36V 7/14A, these work together
Black bike .... (as above) ....
Fault demo sequence (test 1 of 2, new battery with black bike):
1) Battery 456 is not fitted, charge indicator shows 3 green lights, then battery is switched off.
2) Fit battery to black bike, switch it on, its charge indicator shows 3 green lights.
3) Bike display is off, press On button for 1 second, display flickers then stays off, expected it to stay on.
Test 2 of 2, older battery with black bike:
1) Battery 123 ....
As you're new to electrics the charge indicator says as much as a meter would without giving the shop a reason to quibble. You could try out the meter once you've written agreement to a replacement.
Summary: charged battery seems dead because bike display doesn't come on.
Blue bike : battery 36V 15Ah serial 123, controller 36V 7/14A, these work together
Black bike .... (as above) ....
Fault demo sequence (test 1 of 2, new battery with black bike):
1) Battery 456 is not fitted, charge indicator shows 3 green lights, then battery is switched off.
2) Fit battery to black bike, switch it on, its charge indicator shows 3 green lights.
3) Bike display is off, press On button for 1 second, display flickers then stays off, expected it to stay on.
Test 2 of 2, older battery with black bike:
1) Battery 123 ....
As you're new to electrics the charge indicator says as much as a meter would without giving the shop a reason to quibble. You could try out the meter once you've written agreement to a replacement.