I got a microSD Sandisk class 10 card. It's 64 GB, which might be a problem, but I managed to format it to FAT32 using MiniTool Partition Wizard. I used the instructions on Recalbox website to download the newest image and then "burn" it using Etcher.

After firing up the Pi 3 B+ with the card, All I got was a black screen with a yellow lighning bolt in the top right corner, and noting happened. The Red light on the Pi kept flashing like 4 quick flashes, then 4 slow flashes.

The SD card was weird, after I tried reading it again with my PC, it showed a tiny 64 MB (not GB) partition with some files, then a lot of unalocated space.

So I formatted the card and tried using the legacy method of just unzipping the Recalbox OS on the card. Now my Pi displayed like a big RGB pallete square and also that yellow lighning. The red light keeps flashing as the last time.

I heard the power adapter might be a problem. It's an adapted supplied by the seeler and it's 3.1 A.

Any ideas what else I can try?