How to avoid table cell not accessible

I cannot put the cursor in some of the table cells

Read these guidelines and ask a good question. And don’t make it a wiki post unless you understand what you do, which is clearly not the case.

If you’re talking about Calc, it can depend on a protection.
Maybe you’re using a file produced from other people. However, if he does not used a password, just try to unprotect the area selecting
Tools → Protect Spreadsheet

If it requires password you can try with Enter Key for an empty password.