Nor should it be, it is not correct. “T-shirt” uses a dash, but “tee shirt” does not. https://en.oxforddictionaries.com/definition/t-shirt https://www.merriam-webster.com/dictionary/T-shirt
Yes, still some people may still be confused since "usted" also uses " necesita". So here is a link for them to decide which form to use for you:
Philip, techanically "T-shirt" should be written with an uppercase 'T', since the name comes from the shape of the letter.
But Duolingo ususally doesn't worry about casing, so you probably had a different mistake in your sentence. Maybe you wrote "t shirt", without a hyphen.
Verb conjugation does not change for gender.
yo necesito = I need
tú necesitas = you need (familiar singular form)
él necesita = he needs; ella necesita = she needs; usted necesita = you need (formal singular form in Spain and used in Latin America for familiar as well in some places that don't use tú)
All articles and adjectives describing camiseta would need to match the noun for gender.
Every noun in Spanish is grammatically either masculine or feminine and articles and adjectives must match each noun for gender (masculine or feminine) as well as for number (singular or plural). The form "una" is only used with a noun that happens to be feminine in Spanish. Most nouns that end in a are feminine, but you will need to memorize the gender of each noun along with its spelling. Nouns that end in o are usually masculine. I recommend memorizing an article or adjective with a noun to remember its gender.
The masculine singular articles are also used with a noun that starts with a tonic a (The stress is on that first syllable.) So it is "el agua fría" for "the cold water" (Notice that they are avoiding two strong a sounds together, "la" with "agua" would be awkward, so you must use "el", but the adjective "fría" must be in feminine form, because "agua" is feminine.) and "un águila calva" is "the bald eagle".
You can also look up a word in the dictionary to find its gender.
https://dictionary.reverso.net/spanish-english/camiseta The Spanish word has sf in red after it for "sustantivo feminino" which means "feminine noun". There are also examples of use with the article.
https://dictionary.reverso.net/english-spanish/eagle When looking up an English word for its translation to Spanish, the red n before the word stands for noun while the red f after the noun stands for feminine, yet the example shows that it is "un águila". Plural articles have s to break up the two a sounds, so they revert back to feminine plural form for "las águilas" and "las aguas".
You do realize that the shirt was named after the shape of the T, right? https://www.merriam-webster.com/dictionary/T-shirt