Torah has the highest authority on all books in Judaism inclusing Talmud and Mishnah. Can someone quote me a single verse in the Torah where it is clearly stated that one's mother has to be a Jew if one claims to be a Jew otherwise that person cannot convert to Judaism.
By Torah, I assume you mean to include Tanach more generally (as it is contrasted with the Mishna and Talmud).
In that case, Ezra 10:2-3 is pretty explicit. Josh Waxman elaborates on that here.
ב וַיַּעַן שְׁכַנְיָה בֶן-יְחִיאֵל מִבְּנֵי עולם (עֵילָם), וַיֹּאמֶר לְעֶזְרָא--אֲנַחְנוּ מָעַלְנוּ בֵאלֹהֵינוּ, וַנֹּשֶׁב נָשִׁים נָכְרִיּוֹת מֵעַמֵּי הָאָרֶץ; וְעַתָּה יֵשׁ-מִקְוֶה לְיִשְׂרָאֵל, עַל-זֹאת
2. And Shecaniah the son of Jehiel, one of the sons of Elam, answered and said unto Ezra: 'We have broken faith with our God, and have married foreign women of the peoples of the land; yet now there is hope for Israel concerning this thing.
ג וְעַתָּה נִכְרָת-בְּרִית לֵאלֹהֵינוּ לְהוֹצִיא כָל-נָשִׁים וְהַנּוֹלָד מֵהֶם, בַּעֲצַת אֲדֹנָי, וְהַחֲרֵדִים, בְּמִצְוַת אֱלֹהֵינוּ; וְכַתּוֹרָה, יֵעָשֶׂה
3. Now therefore let us make a covenant with our God to put away all the wives, and such as are born of them, according to the counsel of the LORD, and of those that tremble at the commandment of our God; and let it be done according to the law.
I would also point out that the technically correct statement about the Torah (the 5 books) is that it doesn't make an explicit statement either way, thus we are left to derivations from verses, so any claim about the status of someone with only one parent who is Jewish would be subject to the same challenge.
However, straying from the Halachic interpretation of the verses, I would also point you to Leviticus 24:10-23, where we have someone who has a non-Jewish father and is treated as a Jew. Now the Rabbis have a problem with such a reading because in Halacha matrilineal decent only started with the giving of the Torah and thus read the passage as this person was more of a convert, like other Egyptians who joined the Jews when they left Egypt, and that does fit in the passage as a whole. But absent that problem, a straight forward reading is that his mother's Jewish status was sufficient to regard him as a Jew.
In regards to the final comment "otherwise that person cannot convert to Judaism" Judaism does not believe that. Conversion is a basic part of Judaism, and if one is not Jewish by birth (by having a Jewish mother) then one can convert. The passage I quoted from Leviticus is just one among many that speaks about the convert.
First see Talmud Bavli Yebamot 17a (which examined the source in Torah):
"your son"(13) implies that he who is descended from an Israelitish woman may be called thy son, but thy son who is descended from a heathen woman is not called your son but her son.(14)
(13) Deuteronomy VII, "For they will turn away your son from following me, that they may serve other gods; so will the anger of the Lord be kindled against you, and destroy you speedily." (14) I.e., is regarded as a perfect heathen and his betrothal has no validity.