The 50-event threshold
Meta ad sets go through a learning phase while delivery calibrates, and Meta advises structuring ad sets to achieve at least 50 optimization events over a 7-day period. An ad set unlikely to reach that is flagged learning limited. Meta lists the usual causes as small audience size, low budget, low bid or cost control, high auction overlap, an infrequent optimization event, or running too many ads at once.
Not the same as Google's learning period
Worth separating the two, because they get conflated. Google's learning period is triggered by specific documented changes (a new bid strategy, a bid strategy setting change, a composition change) and calibrates over up to three weeks. Meta's learning phase is a volume problem: it is about whether the ad set can get ~50 events a week at all. Meta's own remedy for learning limited is to pick a more frequent optimization event, which is the fix we cover in don't optimize a new Meta campaign for purchases.