doing a ceasar cipher excel project (need help with vlookup!)

My project entails that

Prompt the user to enter a translation factor (may be a positive or negative translation of up to 25 characters). Write a formula to verify that a valid translation factor has been entered. Use the IF function to check that it is between -25 and 25.

Create a translation table to convert each letter of the alphabet to a numeric, then use a formula to apply the translation to each numeric value. Your table should have 3 columns. Suppose the translation factor is 3. Use a formula with the MOD function to create Col 3 of the table.

Col 1 Col 2 Col 3
(Numerics) (Alphabet) (Translated Numeric)
0 A 3
1 B 4
2 C 5
3 D 6
. . .
. . .
Z 2


Prompt the user to enter a plaintext message of up to 7 letters (one character per cell). Use a formula to encrypt the plaintext (your formula will need to use two VLOOKUP functions).

To do the encryption,
Find the plaintext letter in the middle column of the table
Look up its translated numeric (use VLOOKUP) in Col 3
Find the translated numeric in Col 1
Look up its alphabetic in Col 2 (use VLOOKUP)

Decrypt the ciphertext generated in Step 3 to get back the plaintext. Insert additional columns into the translation table, as needed.

this is my first time doing an excel project, so alittle lost.
looks like i got the translation correct with
=IF(AND(B7>=-25,B7<=25),"OK","ERROR")

for the translation i used =MOD(A17+$B$7,26)

as for the caesar cipher translation. i think my formula is alittle off..
14-2.png and i need to use a formula to translate I T W A S to the cipher.. and then back to original letters (see document attached)
im using =VLOOKUP(C10,$A$10:$B$35,2) for the cipher. (not sure if this is right?) and then =VLOOKUP(H15,$B$10:$D$35,3) to go back to the original.. which works with I.. but none of the others. when i edit something in the formula it turns to N.A (help!)