Hasty generalization
Hasty generalization is when you decide something is true for everyone (or always) based on just a few examples. A small or unrepresentative sample isn't enough to support a broad conclusion. The same idea appears as the cognitive bias of insensitivity to sample size: we tend to treat small samples as more reliable than they are. To avoid the fallacy, you need more data or a proper sample before generalising. Related: Insensitivity to sample size.
Examples
Two of my friends got sick after eating at the new café, so that café makes people sick.
The first three applicants were weak, so we'll get no good candidates.
I've had two bad dates with people from that app, so everyone on that app is awful.
Three countries that raised the minimum wage saw job losses, so raising the minimum wage always costs jobs.
The first two episodes were boring, so the whole series is bad.