I've worked in the IT sector for over twenty years.
This sentence is correct. LanguageTool says it's a possible typo ("Did you mean "the" or "IT").
Besides being obviously correct, Oxford Business English Plus has two samples sentences that use this construction (that LanguageTool also considers possible typos):
The telecoms sector fell by 1% and the IT sector fell by 2.5%.
The IT sector has been hit by the bursting of the dot-com bubble.