So I want to write "I love you" but I see "aishitemasu" uses kanji characters for "ai" 愛 and I'm trying to stay away from kanji at the moment. Can I use あい instead?
The short answer is "yes you can." You can always write every Japanese sentence only in hiragana if you don't care how it would look like.
But what's the reason for doing so? If you are a total beginner and don't want to use characters you are not familiar with, then okay, that's one of the valid reasons to do so; sticking to hiragana might imply you wrote this sentence sincerely, not having copied it from somewhere.
A native Japanese speaker who knows many kanji might replace 愛 with あい intentionally to make the sentence look childish, informal, girly, or less serious, for example. Using no kanji might somehow be similar to using no capital characters in English (e.g., Saying "i love u" instead of "I love you").