How can one reconcile Allah's saying: "There shall be no compulsion in [acceptance of] the religion" with His saying: "And slay the polytheists wherever you find them," and is this considered a contradiction?

There is no contradiction between negating compulsion in religion and the command to fight the polytheists. Fighting is not to force them into Islam; otherwise, Jews and Christians would have been compelled when they submitted to the authority of the Muslims, which historically did not happen. Fighting is for two reasons: First: to repel those who attack Muslims and seek to spread the influence of disbelief. Second: to fight those who hinder people from the religion of Allah and prevent the call to it. The Sunnah has specified those who are commanded to be fought, such as aggressors. The Prophet, peace and blessings be upon him, commanded fighting people until they testify that there is no god but Allah, establish prayer, and pay zakat, and what is meant by this is fighting aggressors. This is indicated by the Prophet's advice to his commanders to invite the enemy to Islam or jizya before fighting. Jizya is taken from every disbeliever, and whoever pays it is not compelled to enter the religion.