Actually the sound /ð/ exists in Spanish, but it is an allophone of /d/. For example, the word dedo is pronounced as /'deðo/. That means that for a native Spanish speaker it is very difficult to learn to separate both sounds.

Taken from Wikipedia https://en.wikipedia.org/wiki/Spanish_phonology#Consonants

"The phonemes /b/, /d/, and /ɡ/ are realized as approximants (namely [β̞, ð̞, ɣ˕], hereafter represented without the downtacks) or fricatives[6] in all places except after a pause, after a nasal consonant, or—in the case of /d/—after a lateral consonant; in such contexts they are realized as voiced stops.[7] (In one region of Spain, the area around Madrid, word-final /d/ is sometimes pronounced [θ] especially in a colloquial pronunciation of its name, Madriz ([maˈðɾiθ]).[8]) "