For some reason, when i'm trying to set the thermostat mode, the only option there is "click to set" .
Looking at the legacy rule machine, it lets me set the mode just fine if i try editing an existing automation but can't create new rules.
This is a "bug," but it's a problem with the driver and not Rule Machine. From what I can tell with a quick glance at the code, it does not set the supportedThermostatModes attribute, whch Rule Machine (and other apps) can to read in order to know what options to present. You can verify this by seeing if you see "supportedThermostatModes" under "Current States" on the device detail page; I suspect it won't be there.
If you are comfortable editing the code, you should be able to do this yourself. If the author is still around and is maintaining the code, they should also be able to do it. The developer docs contain information about what commands and attributes are required of the Thermostat capability, which you or they may want to reference: Driver Capability List | Hubitat Documentation. Often, developers post about their code in a thread on the forum, so and going there may be best if you go with the second option (contacting the author). Perhaps that is where you found the code in the first place.
This particular attribute (plus the one for supported fan modes) was recently clarified in terms of the format that is expected for the value (a JSON list, whereas a string representation of a Groovy list may have been used for this with some drivers in the past). That's half the story. In your case, the attribute seems to be missing entirely from my quick look at the driver code, though again, you can verify this on the device detail page. That much was probably always a problem, as I think it was always required by the capability, or at least I don't ever remember it being added; it's just that now, more apps are starting to actually use it (so you only see appropriate options). Rule Machine Legacy apparently did not.
Code updated on Github, new release will soon be out. Wonder if this capability will help with a bug the driver was having when trying to set temperatures with Google Home?
Show the Current States of your thermostat device. If you don't see supportedThermostatModes, or if the modes shown are not in quotes, it won't work. But there is an easy fix.