I had the same problem while trying to use chips.
The scheduler-card needs some ha- elements that are not loaded by default. This is how they fixed it:
Add this file to the project. The elements in the first if statements are to check if the needed elements are already present.
https://github.com/nielsfaber/scheduler-card/blob/5b94a56b8f02c01b6922b81ba927fc391bb4f48d/src/load-ha-form.js
The function is called when creating the element you are working on.
https://github.com/nielsfaber/scheduler-card/blob/5b94a56b8f02c01b6922b81ba927fc391bb4f48d/src/components/variable-slider.ts#L41C5-L41C40
This makes more elements available. Such as ha-checkbox, ha-chip and hass-tabs-subpage-data-table.
Before this fix I had 239 available elements and after this fix 330.
It does not work for everything. The element ha-chip-set is a container for the ha-chip, but it is still not available after this code is ran.
I hope this helps.