For example, 'enfermo' is to be pronounced as 'enfedmo' for me.

The letter 'r' in Spanish words sound very much like the English letter 'd'.

But the real 'r' sound is really difficult to make.

Since r is difficult to make, I can just simply pronounce it as 'd'.

Sorry

The title of this post is incomplete. It's "I think I can simply pronounce the letter "r" in any Spanish word as 'd' ".

The letter "r" in Spanish words pronounces greatly differently from "d".

Therefore, native Spanish speakers will probably misunderstand you if you pronounce "r" as [d].