Are you following the built in checklist and using the auto-FO? This is either not setting a thrust reference or an incorrect bleed transition/setting.
It should be clear when ATS is engaged as there is the mode status display in front of you tells you what it is doing. Green top row for modes and yellow bottom for cautions and disengagement. It is a basic autothrottle and more of a bolt on. It can essentially only do N1 and Speed.
The PFD speed bug is either cyan when the FCP knob has set the speed (i.e. you) or magenta when the FMS is controlling it.
Your hardware throttles do not match the position of the in sim throttles. Move your throttle to 'catch' the in sim throttles.
What vertical mode are you climbing in? The usual departure method is to bug 200 kts, complete the after take off checks (flap up being key) then accelerate to 250 kts. The climb schedule is either 250/250/0.72 or 250/300/0.78 for <10k alt/>10k alt/mach changeover. It's listed in the documents included in the aircraft folder.
For a foolproof but steep climb use FLC and you won't stall.