Why was Paul accused of not being a real apostle?

Paul was accused of not being a genuine apostle because he had not shared the Twelve's three years of close experience with Jesus, and critics claimed he had never even seen Him. Paul answered that he had seen the resurrected Lord and that the believers themselves were proof of his apostleship.

Les teaches that Paul was constantly accused of not being what he claimed to be, because he had not had that three years of experience that the Twelve had. They had been with Jesus day and night for three years; Paul had not, and they said he had never even seen Him. But Paul responds that he did see the resurrected Lord, and he says he will point that out more clearly in a coming verse. He tells them, 'You are my proof of apostleship.' Les explains that it was not just Peter and the Eleven whom the Lord used; He also used Paul, Barnabas, and a little later Silas. Paul is still dealing with his apostleship, and he goes on to show from the Scriptures that a servant of God is worthy of his labor.

1 Corinthians 9:1

Am I not an apostle? am I not free? have I not seen Jesus Christ our Lord? are not ye my work in the Lord?

What was the main complaint against Paul's apostleship, especially from the Jewish community?

The main complaint was that Paul was not one of the Twelve apostles. Critics, especially in the Jewish community, argued he had not walked with Jesus for three years as Peter did, so his apostleship was questioned. He answered that those saved out of idolatry through his ministry were proof.