Is committing some errors, such as the absence of a mahram for girls and mixing in charity convoys, a sin that prevents participation in charitable work, knowing that I advise them not to commit those errors?

As long as you do not participate in matters that violate the Sharia (definition), nor assist in them, but rather enjoin good and forbid evil, then there is no blame upon you for participating in charitable work with that association. Rather, you will be rewarded, Allah willing.