template might not exist or might not be accessible by any of the configured

1、网页报错

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Thu Nov 11 16:30:35 CST 2021
There was an unexpected error (type=Internal Server Error, status=500).

2、idea报错

2021-11-11 16:30:35.393 ERROR 9556 --- [nio-8087-exec-2] org.thymeleaf.TemplateEngine             : 
[THYMELEAF][http-nio-8087-exec-2] Exception processing template "index_alt": 
Error resolving template [index_alt], template might not exist or might not be accessible by any of 
the configured Template Resolvers

org.thymeleaf.exceptions.TemplateInputException: Error resolving template [index_alt], 
template might not exist or might not be accessible by any of the configured Template Resolvers

1、报错时的HTML页面如下

2、解决办法后的HTML页面如下

3、原因
我使用的是idea创建的springboot项目，引用了thymeleaf模板，但是我最开始的html页面没有添加thymeleaf的头就报错了。