Site too large
Users with Classic Sites with a large number of pages will not be able to migrate their Site(s) without first making some changes.
(See this page for Google Sites size limits - these have been slowly increasing so it's worth checking even if you've already seen it before)
In addition, I have recently been made aware that the number of pages that can be migrated is currently less than the number of pages that a New Site is permitted to have. So if you are seeing the "Your site exceeds the maximum number of pages allowed in New Sites " message, that will be why.
(At the time of writing - 7 Aug - the maximum number of migrateable pages is 1200, but this may change without notice.
The recommended approach is to split it into multiple subsites. This has to be done by making copies of the original Site, then deleting subsets of pages to get below 1,000 pages and then migrating each sub-site which may then be linked together.
(An example of a linked sub Site can be seen by clicking on 'Sub-Site' in this Site's menu)
See also this thread about size limits. - there are indications the limits may be raised at some point