I've read that building muscles increase the creatinine level is that true? and can it affect the kidney function? thank you

Only in excess: to the point of muscle breakdown, i.e.-rhabdomyolisis. Normal exercise poses no such threat unless there is pre-existing kidney damage.

Muscle mass: Serum creatinine level is dependent on muscle mass and body builders have higher creatinine levels. The estimated glomerular filtration rate takes race into account to adjust for of African Americans having larger muscle mass in general. The formula also adds a factor for men to account for their larger muscle mass than women. However, the rise in serum creatinine due to muscles is not harmful.