Imaam Shaaf'i complained to his teacher of being unable to memorize and his teacher said to cut back on his sins as Allah does not give knowledge to sinners.

What does this mean?

The statement in question is the famous line of Imam Ash-Shafi’ee, “I complained to Wakee about my decreasing memory and he advised me to abstain from sins, and he informed me that knowledge is light and the light of Allāh is not receptive to the sinner.” [1]

This means that sins affect our memory and ability to understand Islam. The more a person grows in taqwa, the more ability Allah gives him to understand and memorize Islamic knowledge. So the solution for a weak memory is a gradual decrease in sins and increase in righteous deeds.

This will open the heart to be receptive to the light of Allah, i.e., Islamic knowledge.

Allah knows best.

Reference:

[1] ‘Awaa-iq ut talab, p. 38