Canto 3 -
Araṇya-kāṇḍa
Chapter 5: Rāma Meets Sage Śarabhaṅga
Text 3.5.28

मामेष वरदो राम ब्रह्मलोकं निनीषति।
जितमुग्रेण तपसा दुष्प्रापमकृतात्मभिः॥

mām eṣa varado rāma brahma-lokaṁ ninīṣati
jitam ugreṇa tapasā duṣprāpam akṛtātmabhiḥ

mām = me; eṣaḥ = this; varadaḥ = bestower of boons to all; rāma = Rāma; brahma-lokam = to Brahmaloka; ninīṣati = wants to take; jitam = which is conquered; ugreṇa = by terrible; tapasā = penances; duṣprāpam = and which cannot be obtained; akṛta-ātmabhiḥ = by those who are not self-controlled.

Rāma, this bestower of boons to all wants to take me to Brahmaloka which is conquered by terrible penances and which cannot be obtained by those who are not self-controlled.

The author describes what Śarabhaṅga told [Lord Rāma]. Indra, the bestower of boons to all, wanted to take him to Brahmaloka.

Then why didn’t he go with Indra? Śarabhaṅga states that in the next verse.