@essieness, yes, it is; or, it is an admissible form. One clue is suggested by Vincentt, having to do with the level of formality or the style you have set in your spellchecker.
The other is that spellcheckers simply do not cover all logical and legitimate forms and extensions of the language. They are a good guide put a poor master. (And of course they won’t help you at all when you’ve written “there” for “their” or even, alas, “per say” for “per se”). Use the Add feature, but use it judiciously. Even if you are positive that you’ve spelled “ecstasy” correctly as “ecstacy,” check the dictionary before you create permanent immunity to the error.
[Edit] Of course you should never use “could’ve” in formal writing, such as in a paper or business letter. Spell it out as “could have.”