I downloaded levitation for mass effect 3 and now I cannot complete side missions.

How are you unable to complete side missions?

I have obtained some items for the side missions and cannot give them to the person eg: I have the heating units but they are not accepted.  They are still listed as not completed   For the book of Plenix I have accepted the mission but the Aethon Cluster does not appear on the galaxy map on the ship.

Citadel: Heating Unit Stabilizers

"Note: This mission cannot be completed before Priority: Tuchanka. "

Irune: Book of Plenix

"Note: This mission cannot be completed before Priority: Tuchanka."

Have you done "Priority: Tuchanka" yet?

The only mission I have not completed in Act one is the cure for the genophage.   This is where the thresher maw destroys the reaper.  Is this true for the cerberus automated turret schematics?   As mentioned I have picked up the schematics and the heating units and they are not accepted, but not the book, I cannot find the galaxy to start the mission..  I completed the mission where the maw destroys the reaper and one goes into act 2 where the citadel is under attack.  The reason I know this is this is the 2nd time  I have gotten to this point.

Thanks,.

Russ

Citadel: Cerberus Automated Turret Schematics

The C-Sec Officer is available only after "Priority: The Citadel II", although it is possible to obtain the schematics first.

Curing the genophage completes "Priority: Tuchanka", so you should be able to complete "Heating Unit Stabilizers" and "Book of Plenix". Once the Citadel attack is thwarted you've completed "The Citadel II", but you should be able to take care of "Cerberus Automated Turret Schematics".

Though it's not one of the missions you've listed I've had a similar issue handing of the ciphers for "Citadel: Cerberus Ciphers"; when this happens I follow the following instructions to get the quest back on track:

"Note: You may be unable to interact with Dellk after buying the ciphers from the Spectre terminal. Just head to any other Presidium level, then go back to the Embassies and he will be selectable again. Alternatively, saving and reloading the game should fix it as well. This doesn't always work however (Xbox/PC)."

thanks for the info but on 2 of the side missions, not the book, I should be able to buy the items at the Spector terminal in the Spector room where the shooting range is.  But those items are not there unless there is another terminal I am not aware of.

Thanks,

Russ

Following sundance31us's link shows this text with regards to acquiring the Book:

Scan the volus homeworld of Irune in the Aru System of the Aethon Cluster to acquire the book. The Aethon Cluster will not appear on the galaxy map until after completing Priority: Tuchanka.

Return to the Presidium Commons in the Citadel after obtaining the item, and speak to the Angry Accountant. You will receive an update on the Citadel Defense Force War Assets.

There is one other location you can access the Spectre Terminal, but it has a 10% mark-up price, and that is on the Normandy, in the shuttle bay, in the Kiosk on the right.  I use that to review what purchases are available, then go to the citadel to those appropriate stores to make actual purchases.