RecruitingPhase 2Drug trialALPINE: Maintenance Letrozole/AbemaciclibThe purpose of this research study is to see if the study drugs abemaciclib and letrozole are effective and safe for participants with estrogen-receptor positive (ER+), mismatch repair proficient, tumor protein p53 (TP53) wild-type endometrial cancer. The names of the study drugs involved in this study are: * Abemaciclib (a type of cyclin-dependent kinase (CDK) inhibitor) * Letrozole (a type of aromatase inhibitor)