You should run Check for errors regularly. This helps to avoid the list growing to a level that can be daunting to fix.
The results of the check are a simple list of data issues that break the GEDCOM data specification in one or more ways. They are categorised as “may cause a problem for kiwitrees”; “may cause a problem for other applications [but will work OK in kiwitrees]”; and less important issues that “may be a mistake in your data”. The most common issues found are broken links between record types (individuals, families, sources, media, etc.).
In each case there is a reasonably clear explanation of the problem and one or more links to the relevant page where you need to correct the problem.
If we look at the first item it says that an individual (I1931) has a link in their data to a family (as a child) F34343434. But there is no such family in that tree. In this case there is only one solution. You need to remove the false reference from that individual. You might also need to create a new correct link to that person’s parent family but that is a different issue. The error is simply a link to a non-existent family. The reference I1931 is a click-able link so use that that to go to that individual’s page. The only way a broken link like this can be fixed is to manually delete it from the raw GEDCOM data. So click on the Edit item in the main menu, then select Edit raw GEDCOM data. This is not a process I normally recommend as manually changing GEDCOM data is often the cause of such errors. But here the problem is clear, and there is no other solution.
This will give you a pop-up window showing the full GEDCOM data for this individual. Select the incorrect FAMC line (highlighted here in red) and either delete it or amend it to the correct family reference, than click save.
The second error listed is slightly more complicated. It also says that says that an individual (I807) has a link in their data to a family (as a child) F34343434. But in this case the family (F27) does exist but does not have the necessary reverse link back to this individual. In cases like this you will first need to decide which half of the linkage has the problem. Do you need to record this person as a child in this family, or do you need to remove the family link from the child? Only you can know which part is wrong in this particular case.
If the problem is with the individual, then the solution is the same as the first error described above. If the problem is with the family use the click-able link (F27) to go to that family page. Here you will want to add the already existing individual to this family. For that use Edit > Change family members.
The pop-up window that opens has it’s own clear help text displayed, so I won’t repeat that here. Don’t forget to click save before you close it!
The third error in the list above says
INDI I9465 has a SOUR link to 1851. 1851 does not exist.
As with the others, the error is fairly self-explanatory. The individual (I9465) has a reference in their data to a source. But the source is referred to simply as “1851”, whereas all sources should have a reference like “S12345”. In this case the correct reference should have been S96 (the UK Census for 1851) in my tree. I suspect I typed “1851” in the source field and didn’t wait the second or so it takes auto-complete to find the correct title for me.
The solution is much the same as the first error, although in this case you can either edit the raw GEDCOM data or you can edit the fact or event containing the incorrect reference using the normal GUI edit screen. It will be easy to find on the individuals page as invalid source references are highlighted in red text.
There are many other errors you might occasionally see but the solutions will all be similar to these.