Tyrant
Eren failed to copy Lensa's Ability for himself even after four consecutive days of "research." He could only conclude that his Sin series-based Ability was not as convenient as he wanted it to be.
He needed to build his foundation in the light element before even thinking about copying her Ability for himself. He managed to obtain and refine Lensa's elemental attainments. But it turned out that they were not enough for him. They had left some blank spots that he needed to fill in himself.
Eren was a bit annoyed but he got over it quickly. He was only trying his luck. And it was not like the Sin series Ability did not work at all. He would need to strengthen his foundation before trying his hand again.
Eren was keen to have Lensa's Ability for himself because of the potential it carried in his eyes. He believed that he could use that Ability more creatively.