Nirguna Brahman or Shuddha chaitanyam is
‘nishkalam, nishkriyam, shantam, niravadyam, niranjanam’(without form, action.. tranquil,limitless and blemishless(Svetashvatara Upanishad VI.19 )
is unmanifest and so it is neither the karta or bhokta and has nothing to do with karma.
(Ashtavakra Gita 15.4
na tvaṃ deho na te deho bhoktā kartā na vā bhavān |
cidrūpo'si sadā sākṣī nirapekṣaḥ sukhaṃ cara || 4 ||)
When the same chaitanyam manifests with its
unscrutable power Maya (trigunathmika),
it is called Maya
avachinna(scoped by) chaitanyam
(conscious ness)responsible for the functions of creation(Brahma), preservation (Vishnu)and dissolution (shiva)
of entire Existence.otherwise known as Ishwara
mAyA is nothing but prakRRiti or nature (mAyAntu prakRitim vidyAt mAyinantu maheswaram) Svet. Up. IV-10;
Let me be born (as many)’ - tad aikshata bahu syAm, prajAyeya" (Ch. Up. VI-2-3)
Hence created Jivas which are Avidya avachinna chaitanyam.
Jiva(vyashti) is the karta and Ishwara(samashti) is the karma phala dhata(nigraham, anugraham)
in accordance to karma)
verse506 SVSSS of HH Adishankara
Verse 511 of svsss of HH Adishankara says
karrmana jayate jantuhu karmanaiva praliyate
karmanaha karyamevaisha
janma mrutyu parampara
Karma is the reason for birth death cycle
BG 7.20
kamais tais tair hrita-jnanah prapadyante ’nya-devatah
tam tam niyamam asthaya prakritya niyatah svaya
carried away by their own nature prakruthi(satva rajas tams composition) one choose their saguna deity and surrender.
BG 7.21
yo yo yāṁ yāṁ tanuṁ bhaktaḥ śhraddhayārchitum ichchhati
tasya tasyāchalāṁ śhraddhāṁ tām eva vidadhāmyaham
HH shankaras commentary
By Swami Gambirananda
Whichever form (of a deity) any devotee wants to worship with faith, that very firm faith of his I strengthen.
Nama Rupa saguna brahaman is resorted for upasana owing to the limitation of antahkarana.
whose effect is a samskara
But HH Shankara says Jnanam cannot be effect of any karma including upasana which is the manasika karma, as the effect caused by karma can only fall under 4 categories uthpathi, Aapthi, Vikruthi, samskruti.