Newb here. Can someone please help me with Herald of Thunder? I can't equip it

You don't need to put the skill on your bar, it's an aura, it just passively works, assuming you've turned it on

Herald skills dont get allocated in skill bar. Once you have it in your skills menu and spirit to fund it. They will activate on their own

When it detracts from your spirit by 30 then that means its active. All the time. You can stack multiple auras if you have more than 30 spirit.

On PC click the Herald of Thunder icon to toggle the spirit buff on/off. On controller hower over the icon and hold square to open buff/aura menu to toggle it on/off.

You need at least 30 spirit to activate HoT.

Herald of thunder is a passive skill, or rather a passive buff. You don’t actually use/cast it yourself. It’s an effect that happen automatically when you kill enemies that are shocked. To shock enemies you either need to do significant lightning damage or use a skill that will shock enemies (like Thunderstorm with shock conduction II support socketed inside). Shocked is a status effect that makes you deal more damage to enemies under the effect. When enemies are shocked they look «electric».

Are one of your two weapon slots taken up by something that automatically consumes spirit such as a rattling sceptre? The skeletons cost spirit to use. Otherwise you usually just need to right click on the HoT picture in the gem socketing screen to activate it. The spirit cost of herald of thunder is 30 spirit fyi as well so if you only have 30, having it successfully activated will drain all of your spirit until you get upgrades via quests or gear

Do you have the dexterity needed to equip it? If not, add dexterity in your skill tree or equipment. Every skill gem will have an attribute requirement, can be intelligence, dexterity or strength (and also level requirement).

When you have required attribute, put it in the skill bar and right click it on the symbol above the gem

Edit: You also need 30 spirit to equip it, first one you get from king in the mist

The enemies you kill have to be shocked in order for it to deal damage

Herald of Thunder is a reservation skill, so you don’t equip it like a normal ability

You need to activate it from the skill menu and reserve the required spirit for it. Once you allocate enough spirit (30 in this case), you can toggle it on and it will stay active automatically.

It won’t appear as a skill you actively press in combat.

Check Int requirements

Also, some of the support gems for those skills cost extra spirit