I´ve already tried in Tools-Autocorrect-While typing, but it didn´t work.
Well it works for me on my Libo 5.3 and 5.4 versions using Linux-Mint-Mate EN-GB. Have you change the version or system recently? What system, version language are you using? The more information you provide, the more we may be able to help.
Try turning off Autocorrect, shut LibO then restart it and turn Autocorrect on again to see if this gives the system a kick. Some users suggest renaming the config file and trying the system again. Rename it back if it does not help.