When using Centiscorch VMAX's "G-Max Centiferno" attack, can I get a Heat Energy from my discard pile?
No, you may not. Heat Energy only counts as a Fire Energy card while it is attached to a Pokemon, not when it is in the discard pile or anywhere else.
Source: TPCi Rules Team (2020-08-13)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH3_174
SWSH3_174
SWSH3_191
SWSH3_191
SWSH3_34
SWSH3_34