It is not possible to run Strapi in development mode within Strapi Cloud as Strapi Cloud uses an ephemeral file system. With this filesystem, anything stored on the filesystem is only temporary and when the next code deploy happens from your git source provider anything on that filesystem will be deleted.
Generally we recommend that you do all content-type development locally, push that into source control, which will automatically trigger deployments into Strapi Cloud.
We also discuss this topic within our Documentation FAQ section here, as this doesn't only apply to Strapi Cloud but is something we advise for users hosting their Strapi applications anywhere.