A gas-service problem has delayed move-ins at 263 Summer St., a former office building being converted into 77 apartments through a Boston housing program, leaving some tenants without access to homes covered by their leases. One tenant, who asked not to be identified, said she signed a lease in July for a Sept. 1 move-in. Six days before she was due to receive her keys, she was told the date had been pushed to Oct. 1. That date later shifted to Oct. 14, which she said was not guaranteed as of the report published Oct. 14. The tenant said she has been staying on an air mattress with her cat and living out of a suitcase. After paying a security deposit and first month’s rent, she has also faced the cost of a sublet and storage while waiting for access to the apartment. Developer Adam Burns attributed the delay to an unforeseen issue with gas service and said he is providing rent credits for the period the apartments are unavailable. The city said it is pressing the developer to support tenants while working to get them into their homes. Housing attorney Benjamin Levine said a missed move-in date could amount to a breach of the lease, depending on its terms. He said a judge could determine that a landlord is responsible for costs tenants incur because of a delay. The tenant said she still wants the apartment, which she found through a low-income housing lottery, but fears losing it and wants a firm move-in date. “I would just like a definite answer,” she said. The delays come as Boston seeks to add housing by turning underused office space into apartments. For renters navigating the city’s tight housing market, a postponed move-in can mean extra expenses and uncertainty even after a lease has been signed.