So, With the latest .18 update they changed some things around. I pretty sure it's an easy fix, I just don't know how.

The error log.

The mod Upgrade Builder and Planner (2.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event upgrade-planner-next::on_configuration_changed
Unknown style green_slot_button
stack traceback:
upgrade-planner-next/upgrade-planner/gui.lua:156: in function 'open_frame'
upgrade-planner-next/control.lua:58: in function 'verify_all_configs'
upgrade-planner-next/control.lua:78: in function 'handler'
stdlib/stdlib/event/event.lua:335: in function 'pcall'
stdlib/stdlib/event/event.lua:355: in function 'dispatch_event'
stdlib/stdlib/event/event.lua:438: in function 'dispatch'
stdlib/stdlib/event/event.lua:88: in function <stdlib/stdlib/event/event.lua:86>
stack traceback:
[C]: in function 'newindex'
__upgrade-planner-next/upgrade-planner/gui.lua:156: in function 'open_frame'
upgrade-planner-next/control.lua:58: in function 'verify_all_configs'
upgrade-planner-next/control.lua:78: in function 'handler'
stdlib/stdlib/event/event.lua:335: in function 'pcall'
stdlib/stdlib/event/event.lua:355: in function 'dispatch_event'
stdlib/stdlib/event/event.lua:438: in function 'dispatch'
stdlib/stdlib/event/event.lua:88: in function <stdlib/stdlib/event/event.lua:86>

So, a fine individual on the forums answered this question for me.

Panderine:
"Try changing in upgrade-planner/gui.lua "green_slot_button" for "slot_button" inside mod zip file"

fixed it like a charm for .18.27

Thanks for the report. Unfortunately there are some other issues with event handling in .18.27 that prevent me from releasing a new version right now.
I will look into it tomorrow or on the weekend.

Fixed in latest version