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Ivan sklyarov 2
why can't I create developer | trial sandbox ?
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_intro_get_dev_account.htm
https://developer.salesforce.com/docs/atlas.en-us.dev_lifecycle.meta/dev_lifecycle/creating_dev_sandbox.htm
Is it a way to create free sandbox to test installation app exchange ?
Sorry for this issue you are encountering.
May I request you please post what kind of difficulty or problem you are facing when creating a sandbox org? If it's clear then we can look into it and can help you accordingly.
Do you want to create a free developer edition account or a sandbox org?
A developer edition account itself is considered as a production org where you will develop your apex code and test it there itself.
But if you want to create a sandbox org who must have a Professional, Enterprise, Unlimited, or Performance Edition org as production org.The reason being is Professional, Enterprise, Unlimited, or Performance Edition org's are the production instances for which you can create a sandbox copy for them to develop and test your Apex code.Once the apex code is developed then you can push your apex code into any of the above production edition org's.
Please let us know if you have any further queries.
Happy to help further.
Thanks,
Nagendra