Can Pakistani citizens work in Sri Lanka with tourist ETA?

No. Pakistani citizens cannot work in Sri Lanka with an online visa. Remember that a tourist ETA (Electronic Travel Authorization) does not allow individuals to work in Sri Lanka.

Sri Lanka tourist ETAs are issued for short-term stays in the country for the purpose of tourism, business, and transit.

Individuals who wish to work in Sri Lanka must obtain another type of visa, which requires a different application process and sets of requirements. You should apply for a visa at the Embassy as well as ensure the rules of immigration to this country.