Apple Watch SE is not functioning on "Low Battery Mode"

If you can, go to Settings -> Battery -> Battery Health. What figure is reported there for the maximum capacity?

Otherwise, battery replacement would cost $99 from Apple in the US (unless you have an AppleCare+ plan on the Watch). That price may vary in other countries.

Regards.

Charge it for a prolonged period of time. If it only functions in low power mode it depends on how old the watch is. If it is the SE first gen, it cannot upgrade to watchOS 11 so I don't think that a replacement battery is worth it for this watch

The battery is reaching or has reached the end of its useful life. Whether it's worth it to go through Apple's replacement process and cost or to put the money toward a new, or newer, Watch, is your decision.

Regards.