The clean out access point is considered private property and is the homeowners responsibility to maintain. The sewer clean out must be raised to grade. Once it has been raised to grade a brass threaded replacement cap is readily available from plumbing supply section of either Ace Hardware, Lowes, or Home Depot and can easily be installed with a small hand wrench. Once it has been raised to grade The Bureau of Utilities could also provide and install a replacement cap however, it would be necessary for the property owner to accept a service charge of $20 or $45. The link below will give you all the needed details on how to get this resolved once it has been raised to grade.
https://www.howardcountymd.gov/public-works/sewer-cleanout-cap-repair.

This issue will be closed in Tell HoCo. If you require more information, please contact the Bureau of Utilities at 410-313-4900.