The job already has transactions tied to it: In the Housecall Pro system, payment can only be taken on a job and not on an estimate. So if a job already has a payment processed or recorded through it, it cannot be converted to an estimate in the system. If the payment was recorded by mistake, you can delete the payment and then convert the job to estimate.

The job has multiple segments: Jobs that require multiple visits or multiple payments on a single project can be broken into segments. Once a job has been split into segments, it cannot be converted back to an estimate. If the job has multiple segments by mistake, you can delete the segments until only the single segment job is left, then convert as usual.

The job is part of a recurring series: If a job is entered in the system as a part of a reoccurring series of jobs, it will not be able to be converted to an estimate. The system recognizes recurring jobs as pre-set appointments for work, such as a weekly clean or 6-month system check. While the jobs can be moved, edited and rescheduled, they can't be converted to an estimate. 

The job was already copied from an estimate, and is linked to it: When a job has been copied over from an original estimate, a copy of that estimate is kept in the system. The estimate and invoice will be linked together with an icon near the top right of each screen to give you easy access between the two. Since an estimate already exists for this job, the job can't be converted to a different estimate. This keeps duplicates from being created. You can always go back to edit the estimate as needed or delete the job that was created.