Is it true that if a male has undescended testes, he is unable to impregnate a woman?

No: Depends on the other testicle and if the undescended testicle has been brought down by surgery at a young age.

Possibly: Undescended testes, regardless of when they are surgically delivered into the scrotum, are abnormal testes and would not be expected to have "normal" spermatogenesis. Consequently, the fertility associated with these is less than normal testes. When 1 testicle is undescended & the other is normal, no significant difference in overall fertility would be anticipated.

If both undescended: Bilateral testicular maldescent is associated with reduced fertility. Men, in whom both testes remain undescended by puberty will be unable to impregnate. There is an outside chance of successfil intra-cytoplasmic sperm or spermatagium injection. Fertility rate for bilateral udt depends upon age of correction, & are reduced. Chances for men impregnating with history of one-sided udt are excellent.