Labor and labour are two spellings of the same noun. The spelling depends on the type of English.
In American English, “manual labor” is standard.
In British English, “manual labour” is correct.
The same difference applies to similar words, such as “behaviour or behavior,” “honor or honour,” “color or colour,” “favorite or favourite,” “favor or favour,” and “humor or humour.”
A QuillBot grammar check can help ensure you’re using one type of English consistently.