“All clothes we have are on sale!”这句话有错吗？

这句话没错,信我的.

像People, police, cattle, clothes, militia等具有总称意义的名词作主语，谓语动词用复数形式。如：
The cattle are kept in his farm.

像family, company, audience, committee, crew, class, team, group, nation, world, government等名词作主语表“集体”概念时视为单数，谓语动词才可以用单数形式；表“个体”概念时仍视为复数，谓语动词用复数形式。如：
His family is very large.
His family are all painting lovers.

所以你这道题里的clothes是属于不存在双向谓语搭配的名词，它只能用复数谓语。

clothes 即使加了s，也可用单数，在这里all clothes 表示的应该是个整体，故谓语用单数,all clothes we have is on sale.

没错，这个句子是一个定语从句
All clothes是先行词，在从句中做宾语，这个时候关系代词that或者which可以省略，完整的句子是 All clothes that/which we have are on sale

应该没错

没有

没有错。