Single-word adverbs like siempre, nunca and a veces sit right before the verb or at the front of the sentence. Longer phrases built with vez - una vez a la semana, dos veces al mes - go at the end. When nunca follows the verb, Spanish requires a no in front of it as well.
Examples
- Siempre desayuno en casa.
- I always have breakfast at home.
- No voy al gimnasio nunca.
- I never go to the gym.
- Salgo con ellos dos veces al mes.
- I go out with them twice a month.
The mistake to avoid
Two negatives are correct here: no ... nunca is normal Spanish, not a mistake.
Where this fits in
Daily routines & habits, Spanish A2: Talk about what you do every day, what time, and how often. Reflexive verbs and frequency adverbs.