Creating slabs of materials

It would be my great pleasure if anybody can give me some clues to solve this problem. Thank you.

Code:
all_matches = []
for substrate in pot_substrates:
matches = list(sa.calculate(film=film,substrate=substrate,elasticity_tensor=tensor,lowest=True))
for m in matches:
m[‘composition’] =str(substrate.composition.reduced_formula)
all_matches.extend(matches)

Code Problem
TypeError Traceback (most recent call last)
Input In [40], in <cell line: 2>()
3 matches = list(sa.calculate(film=film,substrate=substrate,elasticity_tensor=tensor,lowest=True))
4 for m in matches:
----> 5 m[‘composition’] =str(substrate.composition.reduced_formula)
6 all_matches.extend(matches)

TypeError: ‘SubstrateMatch’ object does not support item assignmen