Is it ok to use dental mouth guard if i have 2 missing teeth and unfilled cavities teeth?

Probably: Probably, but it may not stay in place as well, depending on how many teeth remain and are intact. Look forward to getting cavities filled soon.

Depends: If you lost the teeth awhile ago, then there will be shifting and movement of the remaining teeth. This will disallow the use of the mouth guard. This will be especially true if the mouth guard was fabricated awhile ago. If it's one of those "over the counter boil and place" mouth guards, not one made by your dentist, then go buy another and fit it. Definitely get the cavities addressed asap.

Yes but...: If you do indeed have tooth wear and excessive forces on the teeth then the night guard is a nice source of protection. However, i would not delay much the unfilled cavities. I would also not delay much replacing those missing teeth. Those teeth require care as well.

Not recommended: If cavities are not properly treated by a dentist before mouth guard is fabricated, they continue to enlarge. Ultimately, an untreated cavity can lead to tooth loss, then the ill-fitting mouth guard can cause soreness and irritation to both remaining teeth and gums.