According to the WIPO Standard ST.26 [wipo.int] :
"The symbol “t” will be construed as thymine in DNA and uracil in RNA. Uracil in DNA or thymine in RNA is considered a modified nucleotide and must be further described in the feature table as provided by paragraph 19." - see paragraph 14.
So it is necessary for a user to transform each 'u' symbol to a 't' in an RNA sequence in order that it is compliant with WIPO ST.26. WIPO Sequence will perform this task automatically during an ST.25 import and if the RNA sequence is manually created within a project WIPO Sequence will not allow you to continue with the save.