Can I say to manager "I worked on this project to assist in promo" directly?

You need to own the mistake you made. You caused a delay on a project. A week isn’t a lot, but you chose to do this on your own without prioritization direction or informing stakeholders.

Your work time is not your own, and you misallocated it. You ran into friction for this “side project”, and didn’t decide to bail out or defer the side project, you kept going. You will need to determine what work is high value and explain and champion that work, then do it. You going rogue even if you think it’s for an important project isn’t showing good judgement.

You working on a project that your manager didn’t vet as promo worthy, and then delaying a project that was a priority for your team, is a data point that you’re not behaving as an SDE-III. You need to move past this with humility, not defend it.

You should say something like “I’m sorry, I made a serious mid judgement that negatively impacted the team. I was overzealous and really wanted to work on project XFNA. I mistakenly thought I could ramp up and contribute without impact to Project B, which I was committed to. I was wrong, and even when I realized it would take more work than I thought, kept pursuing it without consulting you, our team, or our stakeholders. I do think project XFNA has great potential for impact, but I was irresponsible prioritizing that over committed work, and will not do that again. I will make sure we catch up and hit future milestones on Project B, and after that would like to discuss if I can contribute some time to Project XFNA, or find something else with cross-team impact, to expand my scope”. You should apologize to the rest of your team and take responsibility publicly, too.

If this team doesn’t have scope to get you to SDE-III and you can’t scalably and responsibly create that scope, you need to find a new team.

What was the impact of the 1 week you spent learning the frontend framework?

Did it lead to a 1 week delay in the main project? What happened as a result of that delay?

If the impact is minimal (and make sure everyone agrees with you, it can't just be wishful thinking), I don't think you need to draw extra attention to this. Just focus on what your plan is going forward to address the gap.

@Lee . Thank you!

@Rahul . Thanks. Actually I underestimated the impact of the delay in Project B. As a result of that delay it led to escalation up to Director level in my team and one other team who is dependent on my work. I am working on addressing the gap now. It is like P1 . Situation is a bit political as well as other team is under significant pressure and I see them escalating on other teams as well (Although I don't have the details) .