Hello @momo-fr,
Thank you for contacting Rank Math support.
Rank Math is not fully compatible with Polylang. We have been trying for so long to add compatibility, but the Polylang team doesn’t seem interested, and that’s why we have stopped making any more efforts towards it.
https://github.com/polylang/polylang/pull/373
So, some features might not work, and you should keep the Remove Category Base option disabled to make category URLs accessible.
You can check for the alternatives here:
https://rankmath.com/compatibility/
Please do not hesitate to let us know if you need our assistance with anything else.