What's the suggested solution for the error I'm receiving?

Selected Data Extension [DE Name] cannot be found. Therefore, this selection cannot be opened.

The cause of this error is that DESelect cannot locate one or more of the Data Extensions used in the selection. It can be due to:

• One or more Data Extensions were deleted.

The solution for this problem is to create a copy of the missing Data Extension in its original location, with the same fields and External Key as before. 

Then, DESelect will automatically fix the selection so you can access and modify it if needed.

Check this article to know more.

Cannot insert a NULL value into a non-nullable column.

There is a nullable field mapped to a non-nullable field in the Target Definition step.

Review the Target Data Extension fields looking for red or orange warning signs.

Either change the source field mapped to that target field or modify the definition of the target field to match the field properties. In addition, you can also create an 'Is not empty' filter type on the corresponding field.

You do not have access.

This error is caused most likely because the browser is blocking the DESelect cookies from being used.

You can follow the steps explained in this article.

The user does not have the necessary permissions inside SFMC to use DESelect or some of the DESelect features.

SFMC's recommendation is to retry and open a support ticket if the error persists.

This error can be caused by an incorrect Target DE configuration, such as mapping a field to a target field with a smaller length

Time-out Error.

SFMC's recommendation is to retry and if the error still persists, you may need to divide the selection into smaller selections. To facilitate this process, you can use Waterfall Selections.

In addition, if you run into this error often enough, you can create a case with SFMC to extend your time-out limit.

Read more about optimisation tips here.

Error: Query activity terminated because it has exceeded the transaction log size limit. Reducing query complexity helps prevent this error in many cases.

Your Selection is too complex for SFMC to process due to the high amount of data in your Selected Data Extensions.

Query failed during execution. Error: Invalid object name '_Open_unfiltered._Open_unfiltered'

Related to an SFMC system error. 

Also seen with _Sent, _Bounce, and _Click

Create an alias for the Data Views removing the underscore (“_”) eg. _Open Data View with an alias of Open.

Check this step-by-step guide for further reference.

Error saving the Query field.'fn_' is a reserved word and may not appear in your query.

This word "fn_" (among others) is part of the SFMC Reserved Word Exceptions table which unfortunately can't be removed.

The word pattern is reserved because SFMC does not support the use of user-defined functions as noted in the SQL documentation below:

https://help.salesforce.com/articleView?id=mc_as_sql_reference.htm&type=5

The only workaround for this error is to either:

• Change the field name and avoid the usage of the reserved string
• Create a new field that replicated the value of the original one, so you can use it in the query - this workaround would prevent the modification of the original field, which can be handy if there are integrations defined with it.