Under such circumstances, you may request the secretary of the association to permit you to carry out the routine yoga and other exercises in the terrace.
You can request for a a duplicate key to be given to you so that you can lock the terrace gate/door once you have completed your daily routine activities which may last for 30 minutes to 1 hour.
You may also suggest that you would buy a lock having at least four keys at your expense and handover other keys to the society by retaining one for you for this purpose.
Actually your flat owner has equal rights in the common areas.
The terrace comes under common areas.
He can fight for his rights to the usage of common areas either by himself or for the benefits of his tenants.
The association cannot restrict the owner/tenant from using the common areas because it is meant for common purpose and cannot be restricted by any bye law or even the rule.
You may put pressure on the owner in this regard because you cannot insist the association beyond certain extent.