Why can I not raise certain skills?

Solution 1:

If you're an Infiltrator then Sniper Rifles and Fitness are both unlockable talent for your class, you will get access to them, but not at the start of the game.

To unlock any locked talent, you need to first place a certain number of points in to another talent first.

For the Infiltrator class these are:

• Sniper Rifles is unlocked when your Pistol talent is at 5.

• Fitness is unlocked when your Tactical Armour talent is at 6.

• Dampening is unlocked when your Electronics talent is at 4.

• First Aid is unlocked when your Decryption talent is at 7.

As per the Mass Effect Wikia.

Solution 2:

Certain talents are unlocked by bringing a predecessor talent up to a certain level.

For Infiltrators, your Pistols skill must be at 5 to unlock Sniper Rifles and your Tactical Armor skill must be at 6 to unlock Fitness.

Please see the Infiltrator wiki article for more information.