To evolve Combee into Vespiquen, the key factor is the gender of the Combee:

• Only female Combee can evolve into Vespiquen. Male Combee do not evolve at all

• In the mainline Pokémon games, a female Combee evolves into Vespiquen starting at level 21

• In Pokémon GO, a female Combee evolves into Vespiquen by feeding it 50 Combee candies ; male Combee cannot evolve. Combee finds a candy approximately every 3 km walked

• Female Combee can be identified by a red triangle marking on the lower honeycomb on their face, which distinguishes them from males

• Since about 88-87.5% of Combee are male, finding a female Combee can take some time

Summary

Game| Evolution Condition
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Mainline games| Female Combee evolves into Vespiquen at level 21
Pokémon GO| Female Combee evolves into Vespiquen with 50 candies

Male Combee do not evolve in any game. So to evolve Combee, you must catch a female Combee (look for the red triangle), then either level it up to 21 (mainline games) or feed it 50 candies (Pokémon GO)