If the reopen button cannot be seen on a FocalPoint purchase request, there can be a number of reasons for this, which should be all checked:
The order is no longer in the batch in Dimensions - a completed or closed order cannot be reopened.
The request on FocalPoint does not match the order in Dimensions - this can occur if the order has been edited in Dimensions (for example lines have been removed added or edited in any way)
The order has an 'on hold' status in Dimensions - on hold orders cannot be reopened as the 'hold' status indicates that the order should not be updated.
The Purchase Request type posts as "Use FocalPoint Purchase Request Consolidation" rather than as a Dimensions PO - reopening these is simply not possible by design
If any of the above are true, then unfortunately you will not be able to reopen the Purchase Request until these have been addressed.