Just speaking generally on push-pull tube amps: 0 mA is perfectly safe, unless the bias reading mechanism is severed (very unlikely) and it’s not actually sitting at 0 mA. In any case, you would see the KT88 plates start to glow orange (usually a large oval patch) and then cherry red as your final warning to quickly SHUT DOWN the amp.
The bias comes up slowly, and you should start to see some reading after a minute or 2. It will increase gradually from there before hitting stability (after ~ 10 minutes as per the manual). But looking for something at 15 - 20 seconds is way way too early.
I’d give it a few minutes, then start adjusting the screw to increase bias until you see a non-zero reading. If you go all the way to the spec’d mA bias early on, it will get too hot later on. So start low and gradually increase. Just keep checking and adjusting (iterate) so it doesn’t run away. You’re a human servo lol. If you’re in trouble you will see the plates glow as your warning. After 10 - 15 minutes do a final adjustment and enjoy.
If it’s using a trimpot you don’t want to overuse that mechanism, so don’t make biasing a daily or even weekly occurrence.
I scanned through the manual and it looks like there's just 1 bias adjustment / reading for all 4 KT88 tubes (cumulative). That probably means you need to stick to tightly matched quads.