New micro SD card not working

I made a post months ago on this subreddit talking about my 208in1 games freezing, and i was told to get a new micro sd card. I buy one from tesco, sandisk 64 gigabytes, i put it in the cartridge, put it in my 2DSXL and it then says SD/TF card error. I know nothing about this stuff so any advice would be greatly appreciated.