A Study of TAK-981 Given With Rituximab in Adults With Relapsed or Refractory CD20-Positive Non-Hodgkin Lymphoma

This study is about a medicine called TAK-981 given with rituximab, used to treat adults with relapsed or refractory CD20-positive non-Hodgkin lymphoma. This study has 2 parts. The main aims of the study are: * To check for side effects from treatment with TAK-981 given with rituximab. * To check how much TAK-981 participants can tolerate. * To check if participants with diffuse large B-cell lymphoma or follicular lymphoma respond well to treatment. Participants will receive TAK-981 and rituximab in 21-day cycles. They will continue treatment for about 12 months unless their condition gets worse (disease progression), they cannot tolerate the treatment, or they leave the study for certain reasons.