Below is after correcting API calls on line 150 to C_AddOns.GetAddOnMetadata and on line 58 to C_AddOns.LoadAddOn in DHUD_Mail.lua
Also corrected api call on line 573 to C_Spell.GetSpellInfo in settings\SettingsImpl.lua
Also corrected api call on line 223 to C_SpellBook.GetSpellBookSkillLineInfo in datatrackers\timers\CooldownsTracker.lua

I'm terrible at coding so hopefully this helps.

10x DHUD/datatrackers/timers/CooldownsTracker.lua:224: attempt to perform arithmetic on local 'bookOffset' (a nil value)
[string "@DHUD/datatrackers/timers/CooldownsTracker.lua"]:224: in function findSpellCooldownsToTrack' [string "@DHUD/datatrackers/timers/CooldownsTracker.lua"]:595: in function updateData'
[string "@DHUD/datatrackers/DataTrackerBase.lua"]:129: in function changeTrackingState' [string "@DHUD/datatrackers/DataTrackerBase.lua"]:152: in function addEventListener'
[string "@DHUD/datatrackers/info/SpellCastTracker.lua"]:518: in function startTracking' [string "@DHUD/datatrackers/DataTrackerBase.lua"]:127: in function changeTrackingState'
[string "@DHUD/datatrackers/DataTrackerBase.lua"]:152: in function addEventListener' [string "@DHUD/gui/slots/GuiSlotBase.lua"]:268: in function setCurrentDataTracker'
[string "@DHUD/gui/slots/GuiSlotBase.lua"]:183: in function rescanDataTrackersList' [string "@DHUD/gui/slots/GuiSlotBase.lua"]:173: in function setDataTrackersList'
[string "@DHUD/gui/slots/GuiSlotBase.lua"]:127: in function onDataTrackersSettingChange' [string "@DHUD/gui/slots/GuiSlotBase.lua"]:142: in function setDataTrackerListSetting'
[string "@DHUD/gui/slots/CastBarSlot.lua"]:353: in function init' [string "@DHUD/gui/GuiManager.lua"]:162: in function init'
[string "@DHUD/gui/GUIFrames.lua"]:2273: in function init' [string "@DHUD/DHUD_Main.lua"]:161: in function main'
[string "@DHUD/DHUD_Main.lua"]:173: in function `func'
[string "@DHUD/DHUD_LibClasses.lua"]:68: in function <DHUD/DHUD_LibClasses.lua:68>

Locals:
self =

{
timers = {
}
listeners = {
}
combatEventsFrame = {
}
eventsFrame = Frame {
}
sources = {
}
customTrackersCount = 0
cooldownsSpellIds = {
}
eventDataChanged = {
}
unitId = "player"
customTrackers = {
}
filteredTimers = {
}
eventRegenerationChanged = {
}
eventExistanceChanged = {
}
spellsCastSuccessTime = {
}
hasListeners = true
trackUnitId = "player"
eventDataTimersChanged = {
}
eventUnitChanged = {
}
isTracking = true
}
spellBookItemInfo = nil
spellType = nil
spellId = nil
overrideSpellId = nil
spellData = nil
charges = nil
maxCharges = nil
startTime = nil
duration = nil
isPassive = nil
baseCooldownMs = nil
gcdMs = nil
bookLast = 3
(for index) = 1
(for limit) = 3
(for step) = 1
bookIndex = 1
bookName = {
shouldHide = false
name = "General"
numSpellBookItems = 31
iconID = 136830
isGuild = false
itemIndexOffset = 0
}
bookTexture = nil
bookOffset = nil
bookNumSpells = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to perform arithmetic on local 'bookOffset' (a nil value)"
trackingHelper = {
eventUpdateInFrequent = {
}
eventCombat = {
}
eventSpecialization = {
}
eventsFrame = Frame {
}
eventAttack = {
}
eventTarget = {
}
eventUpdateFrequent = {
}
timerMs = 578804.576000
eventDeath = {
}
isTargetOfTargetAvailable = false
eventPetBattle = {
}
eventTalents = {
}
playerSpecialization = 2
eventEnteringWorld = {
}
eventTargetTarget = {
}
isTargetAvailable = false
eventModifierKeysState = {
}
listeners = {
}
eventVehicleState = {
}
eventUpdateSemiFrequent = {
}
playerClass = "WARRIOR"
eventUpdate = {
}
eventPet = {
}
eventResting = {
}
}