There’s a known situation currently where dev stores specifically created through the ‘Dev Dashboard’ can’t accept any app plans.
There’s been some conflicting information about this being expected behavior or a bug, but hopefully we’ll get an improvement here before long.
The current workaround options would be to make a free plan available in the app for these stores to use, or to create a dev store through the ‘Partners Dashboard’ instead (now called “Client transfer stores”).