Seeing a filename in Files does not prove that your account can open it or create a neighbouring document. Linux applies permissions to the directory and to the file. A useful support report identifies the particular operation that fails before anyone changes access.

Identify the boundary

Right-click the containing folder, choose Properties and inspect Permissions. Record the owner, group and the access available to your account. Group access matters only if your account belongs to that group. A shared folder should retain the access policy agreed by its owner.

Ubuntu distinguishes listing names from accessing the files themselves. A folder that only permits listing may show its contents while preventing you from opening them. A folder that allows access can still prevent creating or deleting entries. The individual file also needs suitable permissions for reading or writing.

Inspect Properties on one affected file as well. Compare a file you can open with one you cannot, keeping their parent folders in view. Record the full location privately: two documents with the same displayed name may sit under different permission boundaries.

Test only the required operation

With the folder owner's agreement, try opening a harmless existing sample. If the intended workflow includes saving new work, create a clearly named disposable test file and reopen it. A successful read does not establish permission to create, rename or delete files.

Ask the owner to adjust the narrow permission that the workflow actually needs. Do not apply unrestricted permissions to the whole home folder or use Change Permissions for Enclosed Files as a diagnostic shortcut; that option affects descendants too.

Retest using the normal account and application. Record whether reading, saving a new file and editing an existing file now work as intended. If Files succeeds but one confined application still fails, preserve that difference: application confinement may be another boundary beyond ordinary file permissions.

Sources: Ubuntu documentation.