"František likes you."
Translation:František tě má rád.
Because it is unnatural. In the unstressed second position we use the short forms, tě in this case. The long forms like tebe are used in other positions.
what about "Frantisek má rád tebe?" i thought tebe could work at the end but it wasn't accepted either.
Added. It stresses you. František likes YOU. = It is you who František likes.
Read the Tips and notes (web only) or some other recommended grammar reading. The Tips and notes for this lesson contain:
'Some of the forms can only appear in the second position in the clause (like the verb particle se). These "clitics" are the forms listed in italics. Using these outside of the second position is an error, except when a higher priority second-place item, such as the verb particle, pushes the pronoun to the right. Using the two-syllable alternatives to the clitics in the second position tends to be emphatic."'