Every time you turn the ignition on, the SRS computer does a power-on self-
test of itself and its circuitry. If anything reports out-of-range or
doesn't report at all, the light will come on.

In short, a fault has developed in your SRS system, which includes the seat
belt pretensioners. The dealer (or qualifed independent garage) will be
able to scan the SRS system and determine what part has set the error.

Until then, there is a possibility that your SRS system is non-functional.
If that's not an issue for you, you could just invest in a bit of black
tape.