You can assign User Access Administrator to that specific User in the Subscription Scope . You can go to Subscription >> IAM >> Add role Assignment and select User Access Administration and then in members select the user you want to give permissions
.
OR
If you want to give only access to locks then you can create a Custom Role following the below steps :
- Go to Subscription>> IAM>> Add>> Add a Custom
Role.

- Give Custom Role Name and select Start from Scratch.

- In Permissions Tab , Click Add Permissions and search for
Microsoft.Autorization
and select it and then again search for
Microsoft.Autorization/Locks
and select the three permissions as
shown below and click on Add:

- In all other tabs let it be default and in last tab i.e.
review+create , click on create. Once its created Go to
Subscriptions >> IAM >> Add >> Add role assignment and search the newly created role and assign it to the user you want .
Note : If you are assigning the permissions on Resource Group Level then you will be only able to manage the locks of the resources present inside the resource group , but if you assign the same permissions on Subscription Level then you can manage locks on resource groups as well as resources.