Cannot convert "started at" column from text to date in Power Query

I want to extract month & year in a new column but it doesn’t seem possible.

This is what the instructions say on the Building A BI App guided project:
" 1. * Create a new column named month_year that takes just the month and year from started_at. Use that column for the trend axis. If you just use the started_at field, it will aggregate the data by calendar month, which isn’t what we want."