Why some members are unable to participate at war?

When I tried to initiate a 10x10 clan war (I was the clan leader), it appears that 2 members of my clan couldn't participate.

Why does that occur?

Solution 1:

There could be two reasons. Either the amount of people in your clan is not a multiple of five, or some of your clan members are ineligible for war.

Not a multiple of five

Clan wars matchmaking will only match clans of similar sizes, and a few members (between 1-4 per clan) may be put into spectator mode when the clans are matched to even out team sizes. The members in spectator mode will be those with the lowest trophy count in their clan (or those who are ineligible for war).

10 vs 10 members is the smallest possible team size for a clan war. Other possible team sizes are: 15 vs 15, 20 vs 20, 25 vs 25, 30 vs 30, 35 vs 35, 40 vs 40, 45 vs 45 and 50 vs 50.

Ineligible for war

Sometimes you will have a clan member that is ineligible for war and cannot participate:

Members who are new to your clan will be ineligible for war if they left their previous clan while a war was in progress. They will become eligible for war again when the previous war is completed. You can check your clan member list to see if any members are ineligible for war, and see when they will be eligible again. If you start the clan war while members are still ineligible, those members will not be able to participate.

Quotes from the "Clan Wars FAQ" page on the wiki.