In homebrew feats, there is the option to grant additional spells slots

<option

value="27"

id="field-spell-modifier-type-spell-slot-increase" >Spell Slot Increase</option>

However, the modifier subtype dropdown, a required field, has no values for this in either the data or subTypes variables. At this point, I'm not 100% which one is more to blame, but they both have similar data in them.

<script>
(function() {

var subTypes = [truncated];

var defaultSubTypes = [];
var typesValue =
0;
var subTypesValue =
0;
var defaultValue =
[];

var typeField = $('#field-spell-modifier-type');
var subTypeField = $('#field-spell-modifier-sub-type');
var defaultTypeField = $('#field-spell-modifier-default-sub-types');

[truncated]</script>

 Steps to reproduce:

Create a homebrew feat.

Add a modifier.

Select Modifier Type - > "Spell Slot Increase"

Attempt to Select Modifier Subtype (a required field). You will be presented with "no matches found

Set fixed value to the number of spell slots intended to grant.

Attempt to save. This will fail.

Hi,
I'm trying to do the same thing.  Was wondering if you've had any response?

Hi. I attempted to do the same and was not able to. I submitted a ticket so we will see. 

Sorry to tell you this, but this has been a bug ever since the homebrew went up. So they had more then one and a half years to fix this, but never did :(

wah.  My DM has given me a feat that because I combined a druidic focus with an arcane focus, and that has given me an extra spell slot for each level.

This is my first D&D adventure/game, so apart from keeping track of spell slots offline, any other tips/tricks to manage them?

I'm playing the Planescape Torment campaign and the DM told me I can get extra slots.
I've tried to setup a custom/homebrew feat giving me those slots, but encountered the error mentioned in this bug report 3 years ago.
I guess this won't be fixed.
Has anyone found a workaround yet?

Current Work around:

Create a spell, for example:  "Fiendish Boon: Additional Warlock Spell Slot" For spell school I did, level 5 and Divination. Casting time is special so it shows up on the character sheet. Spell range is self and also instantaneous. Description is: "You gain one additional Warlock spell slot that recharges on a short rest". I then made it available only for Warlocks.

Next, go to Modifiers and do "Spell Slot Increase" and select Warlock.

Now, create a Feat under the same name: "Fiendish Boon: Additional Warlock Spell Slot" and add the spell. It says "Use" on the character sheet once the spell is active; it does not actually increase your maximum spell slot so maybe there's something wrong there.

But that's the only workaround I've seen so far.

Quote from EldrythKarnstein >>

Currently working on a workaround for one of my players that's a Warlock. Trying to do "Bonus: Spell slots" and see if that works.

 It will not work. There is currently no way to increase the number of spell slots a PC gets. Those modifiers were entered to implement the Epic Boons, however they have not been implemented yet so those modifiers currently do not work. It is on DDB’s planed improvements, but there is unfortunately no news on how long they will take to implement.

I’m sorry I couldn’t give better news, but at least you won’t waste time trying needlessly.

I edited my last post and created a somewhat lengthy workaround. Essentially you create a spell that has the Spell slot increase modifier, add it to a feat that recharges once per short rest and use that.

But as you said, it doesn’t actually do the thing. You could have just added an Action to the feat with that description and a 1ce/SR “limited use” for a checkbox on the character sheet and done it all with descriptive text and no modifier or spell at all. It would “do” the same thing with less work. 

It functions well enough, I could modify it to scale alongside the Warlock's normal spell slot progression if I wanted to. But essentially this is the best workaround I can find. This one doesn't actually have a checkbox; just the "Use" icon that grays out on use. When you select "Short Rest" the feature automatically recharges as well and appears not only in the proper area on the spell list but also under Special actions as well as the Features and Traits menu.

So for what the above posters were looking for, and until this bug gets resolved, this is the closest fix that anyone's stated.