When you upload a paper, as part of the conversion process, the program checks for the most common markup error which is to leave a space between the curly bracket and the markup code letter(s)

e.g. { p instead of {p

when such a mistake is detected, the upload is abandoned and an error message reports left and/or right

Left refers to the opening markup and right to closing markup

If you see this message then:

go back to the copy you have marked up and are trying to upload
search for the error and remove the space between bracket and code
if your editor software permits e.g. google docs editor, then key ctrl-F to bring up a search box and type in
?[space]
to locate the first left space error (or [space]? for a right error)
NB: The programme tells you which types the error(s) is/are but not how many - there could be more than one